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A knowledge of the atmospheric pressure is of the greatest importance for the explanation of the courses of the winds. The explanation of these maps is found in the "Discussion and Analysis of Winds," where constant reference is made to it.

PLATES 15 TO 20 INCLUSIVE.

RELATIVE PREVALENCE OF WINDS, IN SUMMER and Winter, EXPRESSED IN PERCENTAGE.

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PLATE 20. South Temperate Regions, between latitude 25° and 60° south. These six Plates represent the relative prevalence of winds from the different points of the compass in summer and winter, taken as the most marked seasons, and are adapted to the ready comparison and contrast of these seasons. The width of shading of the outer ring, reckoned from the circumference toward the centre, expresses in hundredths of an inch the percentage given in the Tables for the summer; in like manner, the inner belt of shading is used for the winter. The distance of these pairs of limiting circumferences from each other is 30 per cent.; when, therefore, the tabular percentage is in excess of this amount, the irregular contour line that marks the inner limit of the width passes into the next inner space.

Monsoon influences of marked character are vividly depicted in Plate 17 (Hakodade, Nangasaki and Pekin), Plate 18 (Celebes Sea and China Sea), and Plate 19 (Sween Island, Australia), the belts of shading far outstripping their limits, and even overlapping one another in the cases of Port Blair and Colombo, Ceylon. On the contrary, when the bands are symmetrical for the two seasons, these windroses show the absence of any noticeable monsoon influence, as on Plate 16, for Europe, in the cases of Dublin, Greenwich, St. Petersburg, Vladimir, Debreczin and Gorki.

PLATE 21.

PERCENTAGE of Winds for THE FOUR SEasons.

This Plate differs from the preceding only in containing windroses for spring and autumn, and illustrates the general similarity of the former to winter and of the latter to summer.

PLATE 22.

RELATIVE PREVALENCE OF WINDS IN THE UNITED STATES, IN SUMMER AND WINTER, EXPRESSED IN PERCENTAGE.

[Illustrated by Vertical Projection.]

This Plate, somewhat more compact in form, exhibits facts of the same nature as those contained in Plates 15 to 20, the percentage of winds at any place being represented in horizontal widths measured across the vertical bands. It enables one readily to find at what place wind from any particular direction is prevalent, by simply tracing down the column until great breadth is reached.

PLATE 23.

BAROMETRICAL WINDROSES.

This Plate was drawn by the author as an early attempt to illustrate the connection between the rise and fall of the barometer and corresponding changes in the direction of the wind. The width

of the shading at the several points of the compass shows the average rise or fall of the barometer per day while the wind is from those points, the indicating a rise, and the a fall; the two arrows starting from the centre are directed toward the points of maximum and minimum pressure; and a light line indicates the mean of the two. The arrow that springs from the circumference shows the mean annual direction of the wind. In order to compensate for the rare occurrence of winds from some directions, at several of the places, and make the shading more symmetrical, without affecting the principle of the illustration, the mean rise or fall for each point is combined, in several instances, with the two contiguous ones on either side, and the shading is proportioned to the new means thus found.

PLATE 24.

A METEOROLOGICAL CHART FOR OGDENSBURG, N. Y., 1838.

This plate is a suggestive presentation of meteorological facts. Drawn by the author, in January, 1839, it is believed to be the earliest American effort to connect and vividly illustrate the mutual relation between the results of a minute record of the winds, made by the aid of a self-registering vane, and so many as five of the points chiefly noted in the registers of meteorological observers, viz., amount of cloudiness, fall of rain and snow, and fluctuations in the barometer and thermometer. Deductions from this chart occupy pp. 220-227 of the Report of the Regents of the University of the State of New York, for the year 1838. Each of the circles gives a synchronous view, the shading corresponding in position with the wind then prevalent, and by its width indicating the amount of the contrasted element. From each month, arrows radiating from the centre denote the point of compass from which the wind came that was accompanied by a maximum or minimum of rainfall, thermometric fluctuation, etc.

PLATE 25.

VELOCITY CHART.

This illustrates minutely the general results of a series of observations, covering 700 years, and taken at 418 places on the American continent, from 1854 to 1857. The object was to determine what relation the average velocity of the winds, as a whole, and the varying and separate velocity of each particular wind, has to the results, as to direction and prevalence, that are obtained when the variation in velocity is disregarded. The solution of this question was viewed as vital to the correct study of the winds, and therefore of no small importance in the search for the laws of atmospheric circulation.

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This plate shows that the resultants computed by assigning to each wind its own separate velocity differ from those in which the variation in velocity is disregarded, in being about 9° more northerly, and having a magnitude of 26 instead of 23 per cent.; and, further, that the velocity of all winds in the United States, north of latitude 33°, is a little more than seven miles per hour, resulting in a transfer of air in the mean direction of the main current at the rate of 2.0 or 1.7 miles per hour, according as velocity is counted or omitted.

The arrows represented as flying with the atmospheric current indicate the direction of the winds when only the time of their continuance is taken into account; the dotted lines show the result when the element of Velocity is also regarded. The height of the ordinates in the middle column is proportioned to the average velocity of the wind at each season of the year. In the right-hand vertical series of diagrams, the ordinates that terminate in a continuous line show the velocity of the wind in the mean direction, on the supposition that the entire current moves with the foregoing average velocity; while, in contrast, those ordinates that end in the broken (dotted) lines exhibit the result, as to velocity in the mean direction, when to each wind is assigned its own special velocity; when the latter class or ordinates is longer than the former, which is usually the case, the intervening space contains the sign +.

PLATE 26.

DEFLECTING FORCES.

The "S-shaped" curves are divided into twelve parts to denote the path traversed by a particle of air, in each of the months of the year, when subjected to the winds that are found at Amherst, Massachusetts, Easton, Pennsylvania, New York City, Paris and Pekin, which are taken as representative places. In each case is seen the "parallelogram of forces," of which the diagonal represents the monthly resultant, one side one-twelfth of the yearly resultant, and another side the monsoon influence. Near each is gathered a parallel series of arrows to show the position of these monsoon influences relative to each other.

The law of the Monsoon Influences is seen in two facts: 1st. All these places, except Paris, are situated on the western shore of the adjacent oceans, and their monsoon influences are from the southsoutheast in summer, and from the north-northwest in winter; but at Paris, not thus situated, their direction is reversed. And 2d. The monsoon influences at Pekin, which is emphatically in the monsoon region, and at New York, which is near the ocean, are greater than those at the other places which are not thus situated.

In the diagram at the right, in this Plate, representing an aggregate period of 560 years of observation, taken at more than 60 places in the State of New York, the approximate parallelism and equality of the arrows show the permanent character of the winds, and their divergence or inequality their annual mutations; yet the latter are rather apparent than real, since they are due chiefly to the introduction of new stations or discontinuance of old stations, so producing a slight modification of the result, and not indicating any really marked differences in the annual resultants. Two striking instances of diurnal variation in the direction of the wind are given on the lower part of the Plate for Hudson, Ohio, and St. Petersburg, which are easily explained by the proximity of each of these places to a considerable body of water situated north and northwest of them.

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