feet.

+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----

-20

-10

0

10

20

30

40

50

60

70

80

90

100 ----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----- 1100

1.391

1.358

1.326

1.296

1.267

1.239

1.213

1.187

1.163

1.140

1.117

1.096

1.075 1110

1.403

1.370

1.338

1.307

1.278

1.250

1.224

1.198

1.173

1.150

1.127

1.106

1.085 1120

1.416

1.382

1.350

1.319

1.289

1.261

1.235

1.208

1.184

1.160

1.137

1.115

1.094 1130

1.428

1.394

1.361

1.330

1.301

1.272

1.245

1.219

1.194

1.170

1.147

1.125

1.104 1140

1.440

1.406

1.373

1.342

1.312

1.283

1.256

1.229

1.204

1.180

1.157

1.135

1.113

1150

1.453

1.418

1.385

1.353

1.323

1.294

1.267

1.240

1.215

1.191

1.167

1.145

1.123 1160

1.465

1.430

1.397

1.365

1.334

1.305

1.278

1.251

1.225

1.201

1.177

1.154

1.133 1170

1.477

1.442

1.409

1.376

1.345

1.315

1.289

1.261

1.235

1.211

1.187

1.164

1.142 1180

1.489

1.454

1.420

1.388

1.357

1.327

1.299

1.272

1.245

1.221

1.197

1.174

1.152 1190

1.502

1.466

1.432

1.399

1.368

1.338

1.310

1.282

1.256

1.231

1.207

1.183

1.161

1200

1.514

1.478

1.444

1.411

1.379

1.349

1.321

1.293

1.266

1.241

1.217

1.193

1.171 1210

1.526

1.490

1.456

1.422

1.390

1.360

1.332

1.303

1.276

1.251

1.227

1.203

1.180 1220

1.539

1.502

1.467

1.434

1.401

1.371

1.342

1.314

1.288

1.261

1.237

1.212

1.190 1230

1.551

1.514

1.479

1.445

1.413

1.382

1.353

1.324

1.297

1.271

1.247

1.222

1.199 1240

1.563

1.526

1.491

1.457

1.424

1.393

1.364

1.335

1.307

1.281

1.257

1.232

1.209

1250

1.576

1.538

1.502

1.468

1.435

1.404

1.374

1.345

1.317

1.291

1.266

1.242

1.218 1260

1.588

1.550

1.514

1.479

1.446

1.415

1.385

1.356

1.328

1.302

1.276

1.251

1.228 1270

1.600

1.562

1.526

1.491

1.457

1.426

1.396

1.366

1.338

1.312

1.286

1.261

1.237 1280

1.612

1.574

1.538

1.502

1.469

1.437

1.407

1.377

1.348

1.322

1.296

1.271

1.247 1290

1.625

1.586

1.549

1.514

1.480

1.448

1.417

1.387

1.359

1.332

1.306

1.280

1.256

1300

1.637

1.598

1.561

1.525

1.491

1.459

1.428

1.398

1.369

1.342

1.316

1.290

1.266 1310

1.649

1.610

1.573

1.536

1.502

1.470

1.439

1.408

1.379

1.352

1.326

1.300

1.275 1320

1.661

1.622

1.584

1.548

1.513

1.481

1.449

1.419

1.390

1.362

1.336

1.309

1.285 1330

1.674

1.634

1.596

1.559

1.525

1.492

1.460

1.429

1.400

1.372

1.346

1.319

1.294 1340

1.686

1.640

1.608

1.571

1.536

1.503

1.471

1.440

1.410

1.382

1.356

1.329

1.304

1350

1.698

1.658

1.620

1.582

1.547

1.514

1.482

1.450

1.420

1.393

1.366

1.339

1.313 1360

1.710

1.669

1.631

1.593

1.558

1.524

1.492

1.461

1.431

1.403

1.375

1.348

1.323 1870

1.722

1.681

1.643

1.605

1.569

1.535

1.503

1.471

1.441

1.413

1.385

1.358

1.332 1380

1.735

1.693

1.655

1.616

1.581

1.546

1.514

1.482

1.451

1.423

1.395

1.368

1.342 1390

1.747

1.705

1.666

1.628

1.592

1.557

1.524

1.492

1.462

1.433

1.405

1.377

1.351

1400

1.759

1.717

1.678

1.639

1.603

1.568

1.535

1.503

1.472

1.443

1.415

1.387

1.361 1410

1.771

1.729

1.690

1.650

1.614

1.579

1.546

1.513

1.482

1.453

1.425

1.397

1.370 1420

1.783

1.741

1.701

1.662

1.625

1.590

1.556

1.524

1.492

1.463

1.435

1.406

1.380 1430

1.796

1.753

1.713

1.673

1.636

1.601

1.567

1.534

1.503

1.473

1.444

1.416

1.389 1440

1.808

1.765

1.724

1.685

1.047

1.612

1.577

1.545

1.513

1.483

1.454

1.426

1.399

1450

1.820

1.777

1.736

1.696

1.658

1.623

1.588

1.555

1.523

1.493

1.464

1.436

1.408 1460

1.832

1.788

1.748

1.707

1.670

1.633

1.599

1.565

1.533

1.503

1.474

1.445

1.418 1470

1.844

1.800

1.759

1.719

1.081

1.644

1.609

1.576

1.543

1.513

1.484

1.455

1.427 1480

1.857

1.812

1.771

1.730

1.692

1.655

1.620

1.586

1.554

1.523

1.493

1.465

1.437 1490

1.869

1.824

1.782

1.742

1.703

1.666

1.630

1.597

1.564

1.533

1.503

1.474

1.446

1500

1.881

1.836

1.794

1.753

1.714

1.677

1.641

1.607

1.574

1.543

1.513

1.484

1.456 ====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====+=====

APPENDIX A.

SUGGESTIONS TO TEACHERS.

It is the object of this book to lead the student to the independent discovery of the most important facts in our ordinary weather conditions, and of the interrelations of the different weather elements. This practical study having taught something as to the real nature of atmospheric phenomena by actual observation, rapid and substantial progress may be made in the knowledge of the distribution and of the explanation of similar phenomena in other parts of the world, as derived through a study of the text-books. By means of this combination of the two kinds of study, the inductive and the didactic, the advantages of both may be preserved, and the slow progress of the first method and the unsound progress of the second may be avoided. This book is not a text-book, and it therefore does not attempt to give explanations of various phenomena discovered by the cla.s.s. Explanations will, of course, be called for by the scholars, in increasing number as the work progresses, and the larger relations of the study become apparent. It is best, if possible, to leave the more complicated matters (such as the cause of the deflection of the wind from the gradient, of cyclones and anticyclones, etc.) until the subjects can be taken up in detail and fully explained, for instance in the later years of the high school course. It is not advisable to raise such complicated questions in the grammar school work if they can be avoided. The teacher who has a fairly good knowledge of one comprehensive modern text-book of meteorology, such as Davis's _Elementary Meteorology_, will find himself sufficiently well equipped to answer the questions put by the cla.s.s.

The value of the work outlined in this little book can be much increased if the larger applications of the lessons here learned are strongly emphasized. Suggestions along this line have been made in fine print throughout the text, but the examples given may be further extended to the great advantage of the student. Careful attention ought to be given to the formulating and writing out of the generalizations reached by the cla.s.s, for in these written summaries the results are preserved in compact form.

CHAPTER I.

The work outlined in this chapter is adapted to the lower grades in the grammar school. It is a.s.sumed that the pupils have already had some preliminary training in the simplest non-instrumental weather observations, such as can readily be made during the primary school years.

For the convenience of teachers who may desire it, a brief outline of work suited to the primary school grades is here given. It is desirable that even older scholars be given some such training as this before they take up the exercises of Chapter II.

The central idea in this elementary work is to train the children in intelligent weather observation, so that they may come to appreciate what our typical weather changes are; that they may recognize the types as they recur, and may see how each example differs from, or accords with, those that have preceded it. We are all so directly affected by the weather conditions prevailing at any time that even the youngest children are forced, unconsciously to be sure, to take some notice of these changes.

The work of the teacher is, therefore, simply to direct attention to what is already seen.

When the children come to school on some snowy winter day, with a northeast wind, chilling and damp, attention may be called to the need of overshoes and overcoats, to the piling up of the snow in deep drifts at certain places near the school or in the town, while in other places the ground is left bare; to the ease with which s...o...b..a.l.l.s may be made, and to other facts which will very readily suggest themselves. A day or two after such a storm, when the sun is shining bright in a cloudless sky, when there is no wind and the air is dry, cold, and crisp, the contrasts between these two weather types should be brought out. Instead of snow we now have sunshine: instead of a damp, chilling northeaster we now have a calm and the air is dry; s...o...b..a.l.l.s cannot easily be made in the early morning because the snow is frozen hard and is too dry, but towards noon, if the temperature rise high enough, there may be thawing on the tops or sides of the snowdrifts, and there the snow becomes soft enough for s...o...b..lling. Another weather type, often noted during our winter in the central and eastern United States, and strongly contrasted with both of the preceding conditions, is that which brings us a warm, damp, southerly wind, frequently accompanied by heavy rains. As these damp winds blow over snow-covered surfaces they become foggy and the ground is said to "smoke"; the heavy rain rapidly melts the snow; slush and mud make bad walking; rivers and brooks rise rapidly, perhaps overflowing their banks; low-lying places become filled with standing water. These and other features should all be brought out by the teacher, not by telling the cla.s.s of them directly, but by judicious questioning, and they should be contrasted with the conditions which may immediately follow, when the storm has cleared off, and when the low temperatures brought by a cold wave, with its dry northwest wind, have resulted in freezing lakes, rivers, and brooks, and when skating and sliding may be indulged in. Early summer weather conditions, with their characteristic warm spells, c.u.mulus clouds, thunderstorms, and (near the coast) sea breezes, furnish another long list of typical changes that should be just as carefully noted and described as the more striking winter characteristics. Autumn types add further to the list, which might be extended almost indefinitely.

One whole year of the grammar school course may well be given to the observations suggested in Chapter I, provided that there is no need of hastening on to the more advanced work. The advantage of extending the course over a whole school year is great, because such extension gives opportunity for becoming familiar with late summer, autumn, winter, spring, and early summer weather types, and this is far better than attempting to crowd all the work into one short season. The interest of a cla.s.s can easily be kept up throughout a school year by means of a progressive system of observations. It is best to vary the observations from time to time, and to arrange them so that, beginning with the more simple, they shall gradually become more complete and more advanced as the year goes on. Thus, starting with temperature observations alone, these may be continued for one or two weeks before they are supplemented by records of wind direction and velocity. After some practice in the observation of these two weather elements (say during one month), data as to the state of the sky may be added. Cloud observations themselves may well be graded during successive weeks, so that, beginning with the simplest notes concerning amounts of cloudiness, the pupils shall gradually advance to the point of observing, and perhaps even of sketching, the common cloud forms and their changes. Thus an important step will have been taken towards appreciating the need of a standard cloud cla.s.sification, which may be given later.

The addition of records of precipitation completes the list of simple non-instrumental weather observations, and these records, as well as the cloud records, can easily be graded, so that, during successive weeks, every week's work shall be different from that of every other week. In this progression from the simpler to the more complicated observations lies the secret of making the work attractive. Nothing will sooner check interest in the study than the necessity of making exactly the same observations day after day and week after week throughout the year. A graded course of non-instrumental observation, such as is suggested, gives a very practical general knowledge of our common weather types and changes, and of the relations of one weather element to another. The questions asked under the different headings in this chapter are designed to awaken the interest of the scholars, and to call their attention to the more important points of diurnal, cyclonic, and seasonal changes in weather elements. The teacher will readily think of other questions which may be suggested for the consideration of the cla.s.s.

Although the non-instrumental records are of little value for future reference, as compared with the instrumental observations, they should nevertheless be systematically preserved by the cla.s.s in their record books. After discussion of the daily observations made by the different scholars, or by one of their number, the records may be written upon one of the blackboards reserved for this purpose. At the close of the day, or the next morning, the blackboard notes should be entered in a record book kept in the schoolroom. The teacher may guide in the discussion of the observations; may suggest points overlooked by the scholars; may draw comparisons between the weather conditions of other weeks and of other days. This talking over of the observations is most important, as it never fails to bring out much of interest.

CHAPTER II.

This work may usually be begun in the early years of the grammar school course, as soon as the non-instrumental observations have been satisfactorily completed. The scheme of progressive observations already suggested may be followed to advantage in the instrumental work as well as in the non-instrumental. It is often a good plan to have a different scholar a.s.signed to the task of taking the observations every day, or it may be more advisable to divide the work, making one responsible for the temperature observations, another for the precipitation, etc. It is well to have the daily instrumental weather records written upon the blackboard in the schoolroom, as already suggested in the case of the non-instrumental observations. At the end of each day the blackboard data should be entered in a permanent record book by some one of the scholars, and some ingenuity can be exercised in devising the best scheme for keeping this record. The record book should be carefully preserved in the schoolroom, where it may be referred to by the scholars of future years when any unusually severe storm, or a spell of excessively hot or dry weather, or a remarkable cold wave occurs, in order that comparison with past occurrences of a similar kind may be made. It is well to have the record book of large size, and to have each day's record entered across two full pages. On the left-hand page the temperature, pressure, rainfall, wind direction and velocity, etc., may be entered, each observation in its proper column, the number of columns being increased according to the increasing number of observations. The right-hand page may be left for "Remarks." These "Remarks" should include notes of any meteorological phenomena which did not find a place in the columns reserved for the regular observations, _e.g._, occurrence of hail, or frozen rain; damage by lightning, winds, or floods; freezing up of rivers or brooks; interruption of railroad or street-car traffic by snow, etc., and, in general, explanatory comments on the weather conditions. Instructive lessons may be taught as to the relation of the local weather conditions which prevail in the vicinity of the school, and those of other portions of the country, by comments on newspaper despatches concerning gales along the coast or on the lakes, and resulting damage to shipping; of snow blockades and stalled trains; of severe thunderstorms and tornadoes; of hot waves and sunstrokes, or of cold waves and the destruction of crops or fruits by the frost. The scholars should be encouraged to bring into the cla.s.s any comments on such phenomena as may be of interest in the work.

Such of these newspaper clippings as are of the most value may be pasted in the s.p.a.ce reserved for the "Remarks," where they may be referred to by succeeding cla.s.ses; and in this s.p.a.ce also may be pasted at the end of each week the barograph and thermograph sheets, if these instruments are in use at the school.

CHAPTER III.

These observations may usually be profitably undertaken in the later grammar and in the high school years. The instruments described, while all desirable, are by no means all necessary, and no teacher should postpone the establishment of a course in observational meteorology for the reason that a complete set of first-cla.s.s instruments cannot be secured at the start.

If the school is provided with a psychrometer, there will be no need of the ordinary thermometer, because the psychrometer gives the true air temperature. It is well, however, to have both stationary wet and dry-bulb thermometers, in the shelter, for ordinary school use, and also a sling psychrometer for use in the meteorological _field work_ which forms an important part of the more advanced instrumental work in meteorology. The sling psychrometer may, of course, be used simply as an ordinary sling thermometer.

The simple form of mercurial barometer, without vernier and without attached thermometer, described in Chapter II, will be found the best barometer for general school use. The standard barometer, described in this chapter, is too expensive and too complicated to come into extended use in our schools. Full instructions concerning the care, the reading, and the corrections of the standard mercurial barometer are published by the Weather Bureau, and to these instructions teachers who have such an instrument are referred. (See Appendix B.)

The form of table given at the end of this chapter is intended merely as a suggestion, and not as a rigid scheme to be adopted in every school. In using the instruments here described, practice with the maximum and minimum thermometers (in addition to the simpler work of Chapter II) may be given before any attempt is made to have the cla.s.s use the psychrometer. And in using the psychrometer one week may well be given to the determination of the dew-point alone, before the wet and dry-bulb readings are employed to determine the relative humidity. Absolute humidity, which is not referred to in this chapter, may, if the teacher deem it advisable, be added as another weather element for study. A refinement in the notes on the state of the sky is suggested, viz., that cloudiness should be recorded in tenths of the sky covered by clouds. This is an advance over the earlier, less accurate cloud observations, and is in line with such a progressive scheme as has been recommended. This book is not intended to present a rigid scheme of observational work in meteorology, alike for all schools, but rather to make suggestions for the guidance of teachers in laying out such a course as may fit their own cases.

Under the heading _Summary of Observations_ only a few of the most important climatic elements have been noted. The list may easily be extended by the addition of such data as the following: For temperature, mean diurnal range; mean diurnal variability (the mean of the differences between the successive daily means). For humidity, monthly mean absolute humidity. For precipitation, the maximum daily precipitation; the number of rainy and snowy days in every month, the number of clear, fair, and cloudy days in every month; the mean frequency of rainfall in every month (number of rainy days divided by the total number of days); the number of days with thunderstorms, etc.

It is important that the monthly summaries should be discussed in the cla.s.s, and that the scholars should give verbal statements as to the numerical results which they have obtained. In this way the work will have a living interest, which the mere compilation of summaries does not possess.

CHAPTER IV.

The first thing for any teacher to do who intends to establish a course in meteorology is to secure a supply of daily weather maps. Arrangements should be made to have them mailed regularly from the nearest map-publishing station of the Bureau. It is important that the Sat.u.r.day morning map, which is usually not sent to schools, should be included in the set, as the break of two days (Sat.u.r.day and Sunday) in every week seriously interferes with the value of the work that may be done on consecutive maps. The maps should be securely fastened up in the schoolroom or in the hall. It is advisable to keep at least two maps thus on view all the time, in order that the scholars may be able to study the changes from day to day by comparing the last two or three maps with one another. As soon as they are removed from the wall, the maps should be carefully filed away for future reference. They may be conveniently kept in stiff brown paper folders, each month's maps being enclosed in a separate folder, with the name of the month and the year written on the outside. It is a good plan to keep with the file of maps any newspaper clippings referring to notable meteorological phenomena a.s.sociated with the conditions shown on the maps. Thus, newspaper accounts of the damage done by a hurricane along the Atlantic Coast; of the blockades caused by a heavy snowstorm in the Northwest; of tornadoes in the Mississippi Valley; of hot waves and sunstrokes in our larger cities, will serve to enliven the study of the maps, and will also help, in later references to them, to recall interesting points that might otherwise escape the memory. It is true that newspapers are p.r.o.ne to exaggerate, and that they are lamentably inaccurate in their use of meteorological terms; but nevertheless they may often be profitably used in such general studies as these. Besides the complete weather map, the school will need a supply of blank weather maps, as used by the Weather Bureau. These may usually be secured from the nearest map-publishing station at cost price. In the case of the preparation of ill.u.s.trations for permanent cla.s.s use, as suggested later, it is advisable to employ the blank maps used as the base of the Washington daily weather maps, and to be obtained, at cost price, on application to the Chief of the Weather Bureau, Washington, D. C. These are larger in size (16-1/2 by 23-3/4 inches) than the station maps, and the paper is of a better quality. Colored ill.u.s.trations with these Washington blank maps as the base furnish an economical, simple, and effective means of teaching elementary meteorology.

CHAPTER V.

In this chapter a series of six consecutive weather maps is taken as the basis of the work. The study of the weather elements on such a series of maps gives a far clearer understanding of the distribution of these elements and of their relation one with another, than if a far larger number of single maps are studied which do not follow one another in regular sequence. Teachers should add to the map-drawing exercises by giving their cla.s.ses the data from other sets of maps selected from the school files. Summer maps as well as winter ones should be used, in order that too much emphasis may not be laid on winter conditions. The search for the various cities in which Weather Bureau stations are established, involved in the work of entering the data on the blank maps, furnishes excellent practice in geography. This exercise may be varied, and practice in the location of the different States may be given, if the teacher reads out to the cla.s.s the temperatures in different parts of the various States, as, _e.g._, central Arizona, 34; southwestern Tennessee, 30; northern Nevada, 38, etc. As the State names are not given on the Weather Bureau maps, such a method as this will give a very desirable familiarity with the relative positions of the States of the Union. If the school possesses a blackboard outline map of the United States, it may be a good plan, if the cla.s.s is small, to have one of the pupils enter the temperatures and draw the isotherms on the board before the cla.s.s, and to let the others correct him if they see that he is going wrong. As to irregularities in the isotherms, which may cause trouble, the officials of the Weather Bureau vary somewhat among themselves in dealing with such cases, and no definite rules can be laid down to fit all occasions. It is best to select maps with few irregularities at first, and in time experience will show how the exceptional cases may be treated.

By the scheme of coloring isothermal maps, suggested in this chapter, a valuable series of permanent ill.u.s.trations of noteworthy weather types for cla.s.s use can readily be prepared at very slight cost. For the purposes of these colored ill.u.s.trations it is best to use the large scale Washington blank weather maps, as suggested in the preceding chapter, and to have each map mounted on heavy cardboard after it has been colored. By means of this mounting the maps are prevented from tearing, and can be kept smooth and in good condition.

The scheme of coloring may be varied to suit the fancy of the scholars, for the preparation of these permanent ill.u.s.trations may well be intrusted to those of the cla.s.s who are especially interested in the work, and who are skillful in their use of the paint brush. The work is really very simple and needs only ordinary care. As soon as the drawing of isotherms and the coloring of isothermal charts have been sufficiently studied, the teacher should hang up the daily weather map each day (or a blank map with the isotherms for the day drawn on it), and should call attention to the temperature changes from day to day. In this way the facts of actual temperature changes experienced by the cla.s.s will be a.s.sociated with the larger temperature changes shown on the weather maps.

The sections on temperature gradients may be postponed, if the teacher deems it advisable, until other matters of a simpler nature are pa.s.sed.

The idea of rates of change is not an easy one for students to grasp, and it is far better to postpone the consideration of this subject than to involve the cla.s.s in any confusion at this stage. It is a mistake, however, to omit these sections altogether, as a clear conception of the principle of rates is a valuable part of a student's mental equipment. It is a good plan, in the exercises on lines of temperature decrease, to have maps prepared with _faint_ isotherms and _heavy_ lines of temperature decrease, in order to emphasize the idea of change of temperature, in contrast to the idea of constancy of temperature expressed by the isotherms.

CHAPTER VI.

In the work on the wind charts it is essential to proceed very slowly, in order that the best results may be obtained by the pupils. Some of the aberrant wind courses, which complicate the discovery of the cyclonic and anticyclonic spirals, may well be omitted in the case of the younger cla.s.ses, and considerable a.s.sistance in facilitating these discoveries may be given by suggestions as to adding intermediate dotted wind arrows, in sympathy with the observed wind directions. The anticyclonic systems are always much more difficult to discover than the cyclonic, and care should be taken to a.s.sist the cla.s.s in this matter as much as seems necessary.

The questions in the text are merely suggestive, and are by no means as numerous as it would be well to make them in the cla.s.s. The discovery of the spirals will probably be made by degrees. The concise formulation of the facts discovered will furnish excellent basis for exercises in writing.

It is interesting to note that the discussion as to whether the winds blow circularly around, or radially in towards, centers of low pressure, which usually comes up in every cla.s.s in meteorology at some stage in the study, was carried on in a very animated way about the middle of the present century in this country. Two noted American meteorologists, Redfield and Espy, and their respective followers, took opposite sides in this controversy, Redfield maintaining at the start that the winds moved in circles, and Espy maintaining that they followed radial courses. The truth lay between the two.

CHAPTER VII.

The study of atmospheric pressure is not easy, because the pupils cannot perceive the changes in pressure from day to day by their unaided senses.

Especially difficult does this study become if the cla.s.s has not already had some practice in making barometer readings. When this observational work has not preceded the consideration of the isobaric lines of the daily weather maps, the teacher should introduce the subject of pressure very carefully. The experiment with the Torricellian tube will show the cla.s.s something of the reality of atmospheric pressure, and the variations in this pressure from day to day can readily be made apparent by means of a few barometer readings, if time cannot be spared for a regular and continued series of barometer observations. A word may be said as to the correction of barometric readings for local influences, in order to make these readings comparable, if this matter has not been previously met with, but care should be taken not to confuse the younger pupils too much with explanations at this stage of the work. It may be well to omit this point unless it is brought up by some pupil. The questions asked in the text are merely suggestive. They may be added to and varied at the discretion of the teacher.

Lines of pressure-decrease should be drawn on all isobaric charts studied in the cla.s.s, as they are highly instructive. When the isobars are near together, these lines of pressure-decrease may be drawn heavier, to indicate a steeper gradient. The convergence of these lines towards regions of low pressure, and their divergence from regions of high pressure, seen on every map on which these gradients are drawn, emphasizes an important lesson. Before measuring rates of pressure-decrease by means of a scale, considerable practice should be given in the study, by means of the eye, of the rapidity or slowness of decrease of pressure, as shown by the heavier or lighter lines of pressure-decrease. When the broad facts of differing rates are comprehended, then the actual measurement of these rates is a comparatively easy matter. In any case, however, an appreciation of these rates of change is not always readily gained, even by older scholars in the high school. It is, therefore, of prime importance to proceed very slowly indeed at this point, and to have every step fully understood before another step is taken.

The instructions in the text for measuring rates of pressure-decrease are, that these rates shall be recorded as so many hundredths of an inch of change of pressure in one lat.i.tude-degree. This is done for the sake of simplicity. If this rate is expressed in hundredths of an inch of pressure in a quarter of a lat.i.tude-degree of distance, the numerical value is the same as if expressed in millimeters of pressure per lat.i.tude-degree of distance.

CHAPTER VIII.

The fact of the prevalence of different kinds of weather over the country at the same time is of great importance. It should be strongly emphasized by the teacher in the course of the discussion of the maps of weather distribution. Additional exercises of the same sort may be given to advantage, by letting the cla.s.s plot and study the weather signs taken from any current weather map. Instructive lessons may be taught by talking over, in the cla.s.s, the different ways in which people all over the country are affected by the character of the weather that happens to prevail where they are.

CHAPTERS IX-XVIII.

The correlation exercises will, as a whole, teach few entirely new facts to the brighter scholars who have faithfully completed the preceding work in observations and in the construction and study of the daily weather maps. These exercises do, however, lead to detailed examination and to the careful working out of the relations which may have been previously noticed in a general way only. They give the repeated ill.u.s.tration which is necessary in order to impress firmly on the mind the lesson that the weather map has to teach.

It is a good plan to let different scholars work out the problems for different months. The results reached in each case should be discussed in the cla.s.s, and thus each member may have the double advantage of working out his own problem, and of profiting by the work done by his fellows.

Throughout these exercises care should be taken to have weather maps of all months studied. The exercise on the correlation of the velocity of the wind with the pressure cannot be undertaken unless the work on temperature and pressure gradients (Chapters V and VII) has been completed.