54 REPORT — 1871. 



On Wet- and Dnj-hulh Formulce. Bi/ Prof. J. D. Everett, F.R.S.E. 



The author said August, Apjohn, and Regnault have investigated formul£e for 

 determining the dew-point, by calculation, from the temperatures of the dry- and 

 wet-bulb thermometers ; but Regnault's experiments on the specific heat of air 

 were not performed tiU a later date, and all these authors have adopted, in their 

 investio-ations, the value obtained by Delaroche and Berard, which is -267, whereas 

 the correct value is -237. But when this correct value is introduced into Beg- 

 nault's formula, the discrepancies which he foimd to exist between calculation and 

 observation are increased, and amount, on an average, to about 25 per cent, of the 

 difference between wet-bulb temperature and dew-point. August and Apjohn 

 erred in assuming that aU the air which gives heat to the wet bulb (1) falls to the tem- 

 peratm-e of the wet bvdb, and (2) becomes saturated. These two false assumptions 

 would jointly produce no error in the result, if the depressions of temperature in 

 the different portions of air affected were exactly proportional to their increments 

 of vapoiu'- tension, and if some of the air were saturated at the temperature of the 

 wet bulb. But it is probable that, when there is little or no wind, the mass of air 

 which falls sensibly in temperatiu'e is larger than that which receives a sensible 

 accession of vapour, and that, in high wind, the supposition that some of the air 

 has fallen to the temperature of the wet bulb is more nearly fulfilled than the 

 supposition tliat it has taken up enough vapour to saturate it. The effect of radi- 

 ation, which is ignored in the formulse, tends in the same direction as these two 

 inequalities, and all three are roughly compensated by attributing to air a greater 

 specific heat than it actually has. The discrepancies above referred to are thus 

 explained. 



On iJie General Circulation and Distribution of the Atmosphere. 

 By Professor J, D. Evebett, F.B.S.E. 



The object of this paper was to call the attention of meteorologists to a theory 

 which is jointly due to Prof. James Thomson of Belfast, and Mr. Ferrel of Boston, 

 U.S.A., and which gives the only satisfactory account of the grand currents of 

 the atmosphere, and of the distribution of barometric pressure over the earth's 

 sm-face, the irregularities arising from the distribution of land and water being 

 neglected. Independent proofs were also given of some of Mr. Ferrel's results. 



In virtue of the earth's rotation, with angular velocity to, a body, in latitude X, 



moving along the earth's surface with relative linear velocity v, tends to describe 



on the earth's surface a cm-ve concave to the body's right in the northern and to its 



left in the southern hemisphere, the radius of cuvature of the concavity being 



6850 V 



—^ feet, if the velocity is in feet per second. The deflection from a parallel of 



latitude into a great circle is usually negligible in comparison, being represented 

 by the curvatvu-e of a circle of radius RcotanX, where R is the earth's radius. 



To keep the moving body in a gi-eat circle, or in a parallel of latitude, requires 

 a constraining force per unit of mass equal to 2 to sin X . v, which if the foot and 



second be units, is -7^^^-=:^ ; and this formula applies alike to all horizontal directions 

 bboU 



of motion. 



The air over the extra- tropical parts of the earth has, upon the whole, a relative 

 motion towards the east, and therefore presses towards the tropics with a force 

 which can be computed by the above formula, if the eastward velocity at each 

 parallel is known. If v denote this velocity at any parallel, in feet per second, 

 the increase of pressure per degree of latitude at tliat parallel is '0019 v sin X inches 

 of mercury. This is sufficient to account for the ooserved increase of pressure 

 fi-oni the poles to the tropics, which may be roughly stated at '01 inch per degree. 



Between the tropics, the general movement of the air, relative to the earth, is 

 towards the west, and the increase of pressui-e is therefore from the equator towards 

 the tropics. 



If any stratmn of air have less than the average eastward or westward ve- 

 locity (relative to the earth) which prevails through the sti-ata above it, it will 



