Part 3

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Your Vice-President has elsewhere expressed his opinion that monsoon winds, applying the term by liberal construction to signify winds which recur with returning seasons, cannot with {48} any degree of correctness be asserted to prevail in the United States. It is true that the prevailing surface winds of the greater part of the United States come from the western quadrants--that is, between southwest and northwest--and so are in substantial harmony with the general atmospheric circulation as shown by the upper-wind currents of Mount Washington (from the northwest) and Pike's peak (from the southwest). But, apart from the easterly and northeasterly trades on the Florida coast, it appears from the records that in no case for any considerable section of the country do 50 per cent. of the winds blow, for any consecutive number of months, either from any single point or from two neighboring points of the compass. Occasionally, however, the local configuration of the country is such that winds are drawn up or down valleys, and, being diverted from their free and proper direction, the wind in such cases follows the trend of the valley or depression.

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In general your Vice-President would feel inclined to refer only casually to the work proceeding from the Bureau over which he has the honor to preside, but this year has been marked by special researches and investigations of general interest. As the work of investigation has been entrusted to the professors of the Signal Service, due credit should not be refused them from their own official chief.

Special reference should be made to the work of Professor Charles F. Marvin, whose successful experiments on wind pressures and velocities have attracted the attention of experts both in Europe and in this country. Unfortunately there was available only a small sum (about one hundred dollars) for the expense of experiments, but with this petty sum, supplemented by his ingenuity, Professor Marvin has very satisfactorily determined the coëfficients of the various forms of the Robinson anemometer, with which instrument the velocity of the wind is very generally determined. Following these investigations, the Royal Meteorological Society of England reopened the question, which, after a costly set of experiments with results widely differing from those of Professor Marvin, had been considered closed.

The general results of these researches, which are believed to be sufficiently definite for general questions, are not only prized by the scientist, but they are of value to the engineer and the builder. Indeed, to all interested in costly structures or extended works liable to harm from wind pressures, the factor of safety is {49} a matter of no small pecuniary importance. These experiments show that, as was formerly believed to be the case, the wind pressure varies as the square of the velocity of the wind, expressed in miles per hour; but a most important fact has developed, namely, that the pressure in pounds per square foot is equal to the miles of hourly velocity multiplied by 0.004 instead of 0.005, as was formerly assumed.

Professor Marvin was not content with one system of experiments, but he further attacked the problem in a direct manner by a method which checked and verified his experiments with the whirling machine. On the summit of Mount Washington, at an elevation of 6,300 feet, he obtained simultaneously and under the same conditions, by automatic and electrical apparatus, continuous registration of the pressure of the wind in pounds per square foot and of the velocity in miles per hour.

The results thus verified can be considered as conclusive from a general standpoint. The corrections for the Robinson anemometer thus determined from these experiments are comparatively unimportant at low velocities, say from 10 to 15 miles per hour, being only a fraction of a mile per hour. The uncorrected velocities, however, are in all cases too large, and by greater and greater amounts the higher the velocity. At 60 miles per hour the observed velocities are about 12 miles per hour too high, and for an indicated velocity of 90 miles the experiments show that the actual velocity is but a fraction over 69 miles per hour.

The anemometer formula found to satisfy most closely the entire range of experiments has the following form for velocities in miles per hour:

Log. V = 0.509 + 0.9012 log. v.

This difference indicated by the formula may seem small and insignificant, as it is in the case of light winds, but at very high velocities the differences are very great. For instance, an actual velocity of 60 miles per hour may occur at some time in almost any locality of the United States for a few minutes, and even greater velocities are occasionally reported, apart from severe tornadoes. Under the old coëfficients for the Robinson anemometer an actual velocity of 60 miles per hour would have been reported as 77 miles per hour, which under the old factor of 0.005 would mean a pressure of 29.6 pounds per square foot; but when considered with reference to the true velocity of 60 miles, under {50} the new factor of 0.004, the pressure would only be 14.4 pounds per square foot--a reduction of over 50 per cent. from the pressure-values formerly accepted.

Professor Marvin has undertaken to verify, and also to extend to even lower temperatures, the observations of Regnault as to the pressure of aqueous vapor at low temperatures, especial attention being given to temperature conditions from 0° centigrade to -50° centigrade. These observations disclose, below 0° centigrade, small but constant differences from the values assigned by Regnault.

In all this work Professor Marvin has shown such ingenuity of resource, such skill in adapting means to the end, and such deftness in improvising and manufacturing the requisite instruments as have elicited commendation from all who have seen his work and followed his methods. Your Vice-President alludes to this not only to give that credit rightfully due to Professor Marvin, but to illustrate this as a type of the highly important work which is being done in all branches of science here in Washington by young men sometimes illy equipped as to means, and still more illy paid. Men engaged in work of original investigation should receive higher pay than clerks in charge of routine duties; but unfortunately the majority of them do not.

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The work of Professor Hazen in charting tornadoes and in determining their relative frequency and severity is directly in the line of the Geography of the Air.

Great attention had previously been given to this subject by Lieutenant John P. Finley, who, with indefatigable industry, had accumulated an enormous mass of data relative to these violent outbursts of nature's forces. The figures and deductions previously put forth under the authority of the Signal Service having been questioned, the Chief Signal Officer felt obliged, in view of the growing practical importance of the question, as indicated by the great sums annually paid out in the Ohio valley and in the trans-Mississippi region for protection against tornadoes, to reöpen the subject. Instructions of the most conservative character were given to Professor Hazen to determine carefully the prevalence and number of tornadoes in the United States, the areas devastated by them, and the number of lives lost annually. This work was carefully scrutinized during its progress to see that it should be devoid of theory and rest on the solid basis of fact. The results are most assuring to every {51} one, and must serve to allay the unreasonable fears of the inhabitants of the so-called "tornado districts." It appears that there is no part of the United States in which annually more than one square mile of devastation or severe destruction can be expected for each 185,000 square miles, although cases of limited destruction may occur annually for about every 5,000 square miles of area. In no state may destructive tornadoes be expected, on an average, more than once in two years; and the area over which total destruction can be expected is, as shown by the foregoing figures, exceedingly small, even in localities most liable to these violent storms. The annual death casualties from tornadoes have averaged, in the last 18 years, 102 annually; but it is believed that the death rate from lightning is greater than that from tornadoes, since during March to August, 1890, the names of 110 are on record who have lost their lives by lightning, although the data are incomplete, especially as regards the southern states. These statistics cannot be passed by lightly, however, and it is doubtful if in the main they are much in error. By them it appears from five years' record that the average annual death rate by lightning in the United States is 3.8 per million of inhabitants, or 0.2 above the average. In Sweden, for sixty years, the average has been 3.0; in France, for forty-nine years, 3.1; in Baden, for seventeen years, 3.8; and in Prussia, for fifteen years, 4.4 per million.

Other figures, given by a life-insurance agent in St. Louis, which the author claims to have compiled with great care, place the average annual rate of death from lightning in the United States at 206, being more than double the deaths from tornadoes. It must be understood that these figures are not vouched for, and must be very cautiously received, as originating with companies interested pecuniarily in the statistics.

On the whole, therefore, it may be safely assumed that tornadoes are not so destructive to life as thunder-storms.

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Professor Thomas Russell has formulated a method for prediction of cold waves. They always occur after "lows" and before "highs," and different cold waves vary in extent from three "units" to sixty. A "unit" of temperature-fall is taken as a fall of twenty degrees over an area of 50,000 square miles.

The temperature-fall curves in the United States are approximately elliptical in shape. Perfect ellipses represent actual temperature falls with an error not exceeding six degrees in {52} most cases. These fall lines are intersections of planes with a cone which graphically represents the totality of temperature-fall, the contents of the cone being equal to the area of its base multiplied by its altitude, which is the greatest fall in temperature at the center of the cold wave.

A formula has been devised, based on 127 special cases, representing the amount of fall in terms of the amount of barometric depression in a "low," and the amount of excess if a "high," and the density of the isothermal lines in the region.

From proper consideration of the type of low area, shape of isobars, and position of the long axis, definite conclusions can be drawn as to the subsequent shape of the elliptical twenty-degree temperature-fall area and its position.

A method has been devised, also by Professor Russell, for determining the maximum fall of temperature at the center of the cold wave. The maximum fall and extent of fall being known, from suitably prepared tables, the area of twenty-degree fall can be derived. Previously prepared pieces of card-board are laid in the proper position on a map of suitable scale, and lines drawn around them. Between the line representing the twenty-degree fall and the center, the other falls of thirty degrees, forty degrees, etc., are sketched in.

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The foregoing sketch of the geography of the air may appear too superficial and limited for the purposes of this Society, but its further elaboration was impracticable. Indeed, the subject of meteorology could hardly have been touched upon this year had it not been for the courtesy of Professor Russell in placing at my disposal notes upon translations from foreign publications, especially from the German; which publications I have been unable to examine save in a casual way.

The address, as it is, is submitted only in the hope that it may serve, if no other purpose, at least to indicate the great interest which now obtains in the geography of the air, and which manifests itself in the production of meteorological pamphlets and publications too numerous to permit any one charged with important executive duties to examine them all, even in a non-critical way.