November 



1891.J 



SCIENCE. 



273 



If we were to search Ferrel's writings for tbe most impor- 

 tant principle introduced by him into the study of meteorol- 

 ogy, it would be found in the deflective force arising from 

 the earth's rotation, by which all bodies moving on the earth's 

 surface tend to turn to the right in this hemisphere, but to 

 the left in the southern, ft is curious in reading over the 

 general run of meteorological essays to notice how inade- 

 quately the action of this force is considered. In the first 

 place, it is too commonly said to act only on meridional 

 motions; that is, to make a poleward motion run ahead of 

 its initial meridian, or an equatorward motion fall behind; 

 but to have no effect on a motion to the east or west. This 

 is incorrect, for, as Ferrel sho 77s, the deflective force is inde- 

 pendent of the azimuth of motion, and varies only with tlie 

 velocity of motion and the sine of the latitude. In this he 

 was preceded by others, who discussed the mathematical 

 aspects of the question ; but if we except the overlooked arti- 

 cle of Tracy, no one before Ferrel correctly introduced the 

 action of the deflective force into meteorology. It is not 

 simply that a wind tends to turn aside from the gradient, 

 as may be seem by the most elementary inpection of our 

 weather maps; but that, in thus turning aside, it reacts on 

 the distribution of pressure by which its motion is caused, 

 and produces a very significant re-arrangement of pressures 

 in some cases. This was first demonstrated by Ferrel; and 

 if the student wishes to appreciate the conditions under which 

 the winds move, he should follow this subject out to its end. 

 The most conspicuous effect of the re arrangement of press- 

 ures in this manner is the reduction of the polar high press- 

 ures, such as would exist if determined by low temperature 

 alone, into low pressures: for, on account of the earth's rota- 

 tion, the whole system of terrestrial winds in temperate and 

 frigid latitudes runs in a great whirl around the poles from 

 west to east; and the centrifugal force thus developed in ex- 

 cess of that characterizing the rotation of the earth itself, 

 suffices to withhold so much air from the polar regions that 

 the anticipated high pressure due to low temperature cannot 

 occur there: tlie air thus withheld from the polar regions 

 forms a broad belt of high pressure around the tropics. The 

 importance of this even in elementary teaching must be ap- 

 parent; for when a teacher tells his class that the general 

 winds flow because the diS'erence of temperature between 

 the equator and the poles establishes a convectional circula- 

 tion, the class has a right to ask why the region of low tem- 

 perature is not the region of high pressure, as it should be in 

 a convectional circulation. No sufficient answer to this sig- 

 nificant question is to be found in any text-book in our lan- 

 guage, except FerreFs "Popular Treatise." Not only so; 

 some of the most eminent meteorologists give no particular 

 attention to this aspect of the question. For example, in the 

 recent "Report on the Meteorology of the Challenger Expe- 

 dition," the most beautifully illustrated of any meteorologi- 

 cal work ever published, Buchan passes over the matter with- 

 out alluding to Ferret's explanation of it, and without giving 

 any adequate explanation of it himself. In Germany there 

 is a much better appreciation of the nature of the ease, as far 

 as it is represented by the investigations of mathematicians 

 and the discussions in recent text-books. The contrast be- 

 tween the attitude of the conservative British and the pro- 

 gressive German schools may indeed be taken as indicating 

 the dift'erence between the ol.ier and the more modern status 

 of meteorology; the division between the two being on the 

 lines marked out by Ferrel. Certainly, wlien we find tliat 

 the general distribution of atmospheric pressure, the general 

 direction of the greater part of the atmospheric circulation 



and the general velocity of its motion all depend on the 

 deflective forces arising from the earth's rotation, it is not 

 unfair to claim for them and for the investigator who first 

 properly introduced them a large share of credit in the recent 

 advances of meteorology. It is the same with cyclones; 

 those of the torrid regions, where the deflective force is small, 

 present illustrations of distributions of pressure and circula- 

 tion of wind dependent chiefly on dilTerences of temperature 

 and local centrifugal force; but in teniperate latitudes, where 

 the sine of the latitude is of a considerable value, the low 

 pressure of the central part of the cyclonic stoi'ms is in great 

 part the product of outward deflective force that accompanies 

 the motion of the winds Finally, even in the small vor- 

 ticuiar whirls of tornadoes, the deflective force has its eifect; 

 not directly, as in the case with cyclones proper, but indi- 

 rectly: the tornado whirls around because it is developed in 

 a whirling cyclone, and the cyclone turns because it is de- 

 veloped on a rotating earth. Indeed, in following through 

 Ferret's admirable theory of tornadoes, the only theory of 

 tornadoes worthy of a name, it is made clear that if the de- 

 flective force of the earth's rotation were not, indirectly at 

 least, communicated to the tornado, its violence would be 

 greatly reduced, perhaps to the degree of rendering it nearly 

 harmless. 



Thcintroduction of a general principle into ascience, where- 

 by a variety of apparently independent facts are found to be 

 bound together by a comparatively simple relation, is in itself 

 a great contribution to knowledge. The grand views of the 

 correlations that connect all the winds of the world that are 

 gained through Ferrel's essays repay 'the effort needed to 

 study them out to the point of clear understanding; not that 

 the essays are obscure or unnecessarily complex, but that 

 their reading involves a rather clear knowledge of physics 

 and mechanics, not to speak of mathematics, and a careful 

 following of close reasoning from premises to conclusions. 



No just appreciation of Ferrel's simple life and broad sci- 

 entific work can be given in a brief article. His work in 

 meteorology is much more varied than may be inferred from 

 the emphasis here given to a single one of the leading prin- 

 ciples tliat he followed. The others will be found by the 

 faitliful students of his books. His studies in other subjects 

 than meteorology are of sulBcient importance to deserve a 

 separate notice. He was far enough advanced in astronomy, 

 while employed in our Nautical Almanac ofiice, to give new 

 understanding to one of the puzzles of the sky; an unac- 

 counted acceleration of the moon's motion was explained by 

 him as a result of a retardation of the earth's rotation, caused 

 by the action of the tides. The interaction of the lunar and 

 terrestrial tides was also perceived, and when in the Coast 

 Survey office in Washington, the calculation of tide-tables 

 at our Atfantic ports was a subject of advanced .study. A. 

 tide-predicting machine was then devised, by which the labor 

 of thirty or forty men is now saved. Later in the Signal 

 Office, Ferrel prepared his report on "Recent Advances in 

 Meteorology," and gave lectures to the lieutenants on duty 

 there, the substance of these lectures being now published in 

 the " Popular Treatise on the Winds," referred to above. 



Ferrel's simple manner of living kept him apart from the 

 world about him; he had warm friends, but they were com- 

 paratively few. These few unite in feeling that it was a 

 privilege to know such a man; modest, unassuming, even 

 humble in his ways; yet with an insiglit into the truths of 

 nature that goes only with rare genius. lie was one of the 

 small number of men in the world who not only advance 

 the limits of knowledge, but who turn the search for it into 



