mean Pressure of the Atmosphere in different Latitudes. 471 



of the earth; these therefore would conspire to produce a 

 greater mean height in high latitudes than in equatorial, a 

 result the reverse of what we find by observation. The causes 

 therefore which tend to produce it must be counteracted in 

 their effects by an opposite force, which is able, in addition, to 

 sustain the greater equatorial pressure. Such a one may be 

 found in the centrifugal force of the earth's rotation on its axis. 



At the equator the centrifugal force is in direct opposition 

 to that of gravity, but at all latitudes between it and the pole, 

 it may be resolved into two forces, 



the one directly opposed to gravity e 



as de {C being the centre of the 

 earth, and A B its axis), and the 



other in the direction of a tangent 



to the surface of the sphere as B J. A C B 



With the former of these we have 

 here nothing to do, but the effect of the latter is to urge on the 

 atmosphere from the poles towards the equator, so as to cause 

 the air to accumulate in tropical latitudes, until its pressure 

 produces a resistance, sufficient to counterbalance the force of 

 the current. 



We find from the table, that this force is predominant only 

 between the 32nd and e^th degrees of latitudes, and that on 

 both sides of this zone it is more than counteracted by the 

 causes previously mentioned as having an opposite tendency. 

 Now in this zone the tangent force is the greatest, decreasing 

 from latitudes 40° and 50*^ in nearly equal proportions in both 

 directions. 



Without attempting to show by calculation the correspond- 

 ence btween the resistance, caused by the greater tropical 

 pressure of the atmosphere, and this force, for the sake of 

 comparing its different degrees of strength, I have subjoined 

 some calculations of its force at different latitudes. 



The centrifugal force at the equator is equal to a velocity 

 of 0*1 112 foot per second, and decreases in proportion to the 

 cosine of the latitude ; therefore at latitude 60° it is equal to 

 0-500 X 0-11 12 = 0-0556 ; and at any latitude the whole force 

 is to that portion of it which is a tangent to the sphere, as 



, „ 0-500x0-0556 



cotang. to cosme; therefore -zr^~—, — r~c^r.o\ — 0'0481 foot ; 

 ° 0-5773 (cot of 60 ) 



the velocity which this force would give to the air in one se- 

 cond at latitude 60°. 



Again, at 30^ the centrifugal force is 0-866 x 0-1112 



««««„ ,0-866x00933 ^^.^^ r . , 



= 0-0933 ; and ,-^,7^. ~^^ = 0'04;66 foot per second ; 



1-732(00130") ^ ' 



