and Distribution of the Atmosphere. 205 



law, which is a mere summary of observed facts, is stated in the 

 following words in Mr. Buchan's recently published text-book. 



" The wind neither blows round the centre of least pressure 

 as circles, or as tangents to the concentric isobaric curves, nor 

 does it blow directly towards that. centre; but it takes a di- 

 rection intermediate, approaching, however, more nearly to the 

 direction and course of the circular curves than of the radii to 

 the centre ; or the angle is not a right angle, but from about 

 60° to 80°." 



Dove's law of rotation of wind-direction at any one place (viz. 

 that it changes from north to north-east, then to east, and so round 

 with the hands of a watch in the northern hemisphere, and in the 

 opposite direction in the southern) is probably an indirect con- 

 sequence of the same tendency to deviate to the right in the one 

 hemisphere and to the left in the other; but Dove's law is not 

 a law in any strict sense of the word, it is merely a statement of 

 what happens in a majority of instances. 



The quantitative determination of the forces dealt with in the 

 present paper may be obtained as follows ; and it is important to 

 bear in mind that they require no correction for friction. 



Let v denote the horizontal velocity of a moving body relative 

 to the earth's surface, 

 P the constraining force, per unit mass of the body, re- 

 quired to prevent deflection, 

 X the latitude in which the body is, 

 R the earth's radius, about 21 million feet, 



27T 



w the earth's angular velocity, which is Rfl . , if the 



second be the unit of time. This makes 2&>= 7^77^77. 



6850 



I, If the motion be along a meridian, the constraining couple 

 must be equal to the change of angular momentum per unit 

 time; that is, f i 



P . R COS \ = -r- (ft>R 2 cos 2 X), 



whence -r, ~ . . ^dX 



P=— 2o>sinX. R-T7' 

 dt 



= — 2cosinX. v. 



II. If the motion be along a circle of latitude, the excess of 

 the centrifugal force of the moving body above that of a body 

 simply resting on the earth is 



(ft/RcosX-f-p) 2 __ (&>R cos X) 2 

 R cos X R cos X 



2(ollco$X.v + v' 2 ^ / v 



=2covl 1 H 



RcosX \ 2&)Rcos\ > 



