Nov. 23, 1876] 



NATURE 



n 



regions as upper currents, descend to the surface of the globe on 

 the tquatorial side of 30° of lat. in both hemispheres, they then 

 travel onwards towards the belt of equatorial calms, when they 

 meet and ascend into the upper regions of the atmosphere, 

 whence they travel back towards the poles as upper currents, 

 until they arrive at the calm belts of Cancer and of Capricorn, 

 on the polar sides of which they once more descend to the sur- 

 face, and are then known as the westerly winds of the temperate 

 zones. Owing to the rotatory motion of the earth, it is im- 

 possible for these westerly winds to blow direct towards the 

 poles, but it is clear that if you surround the Polar regions with 

 a belt of westerly winds, that no matter what the direction of the 

 •wind may be in the Polar regions, it must, if a surface wind, be' 

 supplied by this zone ; and that the winds experienced in the 

 Polar regions are winds travelling on the surface, and are drawn 

 from this belt of warm winds, is, I think, proved by the fol- 

 lowing extract from an account of the wintering of the Hecla, 

 Capt. (Sir Edward) Parry, at Melville Island, in the year 

 1819-20 : — 



" A gale of wind, from whatever quarter it might blow, was 

 almost invariably found to raise the thermometer several degrees, 

 even when it came from the north, as much as 14°. An east, 

 south-east, or east-south-east wind causes the thermometer to 

 rise 40°." 



From this extract it is evident, as might be supposed, that any 

 current of air from this zone or belt of warm westerly winds 

 raised the thermometer considerably, but that the wind that pro- 

 ceeded the more directly from the ocean and had the least land 

 to traverse was the warmest. 



Maury, without attempting to prove his case, and indeed throw- 

 ing the onus of proof on those who ventured to disagree with him, 

 considers that the south-east trade winds of the southern hemi- 

 sphere become the south-west winds of the north temperate zone, 

 and vice versd ; that the north-east trades of the northern hemi- 

 sphere become the north-west winds of the south temperate 

 zone. I do not say that this is not the case, but if you admit 

 that the north-east and south-east trade winds meet in the belt of 

 equatorial calms and there ascend, it appears to be more reason- 

 able to suppose that their currents intermingle and that their 

 naixed volume is then drawn off north and south as required to 

 restore the equilibrium of the atmosphere. And there is a very 

 strong argument against Maury's hypothesis, viz., that as the 

 south-east trades of the southern hemisphere are stronger and 

 extend over a greater surface than the north-east trades of the 

 northern hemisphere, and as also the north-west winds of the 

 southern hemisphere are stronger and more continuous than 

 the south-west winds of the northern hemisphere, it is illogical 

 to suppose that the stronger polar current, i.e., the south-east 

 trade, feeds the weaker equatorial current, i.e., the south-west 

 winds of the north temperate zone ; it v^ould be more reasonable 

 to suppose the reverse to be the case. 



This entire theory of atmospheric currents is antagonistic to 

 the presumption that a larger body of vapour is carried from the 

 northern to the southern hemisphere. 



Owing to the scarcity of land, and especially of very high land 

 in the south temperate zone, not only is the precipitation less, 

 but the vapour-carrying winds, i.e., the westerly winds, are far 

 more constant in their direction and force than are the westerly 

 winds of the northern hemisphere. (The proportion of westerly 

 winds to any others in the tempera'.e zone in the North Atlantic 

 is two to one, while throughout the south temperate zone they 

 are so constant as to have been christened by Maury the north- 

 west trades. ) 



If you once admit that these westerly winds are equatorial cur- 

 rents flowing towards the poles (a fact susceptible of undoubted 

 proof), it is easy enough to account for the low barometer in 

 :he Antarctic regions, as also the larger amount of precipi- 



ion there as compared with the precipitation in the Arctic 



1. Because the westerly winds being much stronger and more 

 I )iitinuous in the southern than in the northern hemisphere, the 



cension of the air in the South Polar regions must be greater 

 an in the North Polar regions. 



2. Because, owing to the westerly winds of the south tera- 

 rate zone parting with less of their moisture (as previously 



xounted for) than the corresponding winds of the north tem- 



Tate zone, and also to their being stronger and more con- 



nuous, it is evident that when they meet with Antarctic cold 



and their vapour is condensed, the precipitation must be 



qreater, which also involves the giving out of a much larger 



amount of lament heat and the consequent greater expansion and 

 ascension of the atmosphere in the South Polar rejjions. 



It is, I believe, universally acknowledged that all winds must 

 blow from a high to a low barometer, i.e., from a zone of high 

 pressure to a zone of low pressure (not directly, but in a direc- 

 tion modified by the earth's rotatory motion). I may therefore 

 fairly argue that the zones of low pressure at both the equator 

 and the poles proceed from the same causes, i.e., from precipita- 

 tion, and from the ascension of the atmosphere, and that the 

 lower barometer in the South Polar regions fairly accounts for 

 the greater strength and continuity of the westerly winds of the 

 south temperate zone, and that without these constant inequali- 

 ties of pressure we should have neither trades nor westerly 

 winds. 



I have purposely from want of space avoided speaking other- 

 wise than generally of the effect of the land on atmospheric 

 currents, nor is it directly pertinent to my present argument. 



The hypothesis of atmospheric circulation which I have very 

 briefly sketched is in many of its features susceptible of absolute 

 proof, more especially in the following points, viz. — 



1. That the trade-winds descend to the surface of the ocean 

 on the equatorial sides of the calms of Cancer and of Capri- 

 corn. 



2. That the trade-winds ascend in the belt of equatorial 

 calms. 



3. That currents flow from the equator in the upper regions of 

 the atmosphere in an opposite direction to that of the trade-winds 

 on the surface of the ocean. 



4. That these upper currents, flowing from the equator, 

 descend again to the surface of the ocean on the polar sides of 

 the calms of Cancer and of Capricorn. 



5. That these equatorial currents, subsequent to their descent 

 on the polar sides of the calms of Cancer and of Capricorn, are 

 known as the westerly winds of the temperate zones. 



And with reference to my supposition that these westerly 

 winds ascend in the Polar regions, one strong evidence in favour 

 of this is, that if, as I say, the ascension of the atmosphere is 

 greater in the South Polar than in the North Polar regions, 

 the counter or return current towards the equator must also be 

 greater, which is the fact. 



The onus lies on the promoters of the new hypothesis either 

 to reconcile their views with the existing theory of atmospheric 

 circulation or to supply us with a better theory, and one which 

 shall agree equally well with well-established facts. 



October 27 Digby MURRAY 



Deiiniteness and Accuracy 



In my lecture on Force (antS, Sept. 21), I take' for granted 

 that the scientific use of the word is that with which all are 

 familiar in the expression ^^ the parallelogram, of/orces." Hence 

 Newton's term for force is vis impr/ssa (Thomson and Tait's 

 Nat, Phil, § 217) ; though, where there is no room for ifiistake, 

 he often employs the single word vis. 



One of the main objects of my lecture was to protest against 

 the absurd custom of translating the word vis in every case by 

 the scientific v/orA/orce. It is not easy to get an unobjectionable 

 single word for the purpose, for most of the available words have 

 already a semi-scientiho sense attached to them. The word 

 power is very flexible in i's meaninjj^, and would have been suit- 

 able had it not been already seized by the eui^ineers. Thus 

 (Thomson and Tait, § 216) vis insita is rendered innate poiuer. 

 And, giving the word as wide an application as Newton gives to 

 vis, we might render vis viva as active power, which is not far 

 from actual or kinetic energy. But this is merely a suggestion. 



In Pogqeniorff s Annalen (No. 7 of this year) Prof. Zollner 

 translates the scientific term, " the perpetual motion," by " die be- 

 harrliche Bewegung," and thus, to his own satisfaction at least, 

 proves me to be ignorant alike of the proper meaning of the 

 \j%\\n perpctuum mobile ^Mdi of the first law of motion ! 



In another journal I have lately been held up to scorn, 

 not in the main for any real or imputed fault of mine, but because 

 my would-be critic (Mr. R. A. Proctor) happens not to know 

 the scientific meaning of ^^ absolute" measure ! ! 



I could give many more telling instances, great and small, but 

 I have given enough to show how needful was my contention for 

 definiteness and accuracy. P. G. Tait 



College, Edinburgh, November 11 



