536 



Mr. J. A. Broun on the Directions ^c. of [Feb. 1, 



If we remember the increased dimensions of a nnit of mass of the 

 atmosphere as we ascend and take the equation 



where m is the unit of mass, Vj^ the velocitj at a height h, n the number 

 of units in the yertical column in motion, and Ii(mv) the sum of the 

 units of mass in the column into their velocities, then it seems probable 

 that the height h will not be higher than the stratum of the cirro-stratus, 

 nor lower than that of the scud and cumulus. In all probability, then, we 

 shall have (;//) — between 23° and 15°; and there cannot be any con- 

 siderable error in concluding that for the height Ji 



(;//) — 0=+ 20 approximately*. 



It will be remembered that we have already found (p. 526) that 



0-0=+2O°; 



and since (p, the direction of the surface current, appears similarly in this 

 and in the preceding equation, we have approximately t 



Or, That the average direction of the isoharic lines and the mean direction 

 of the mass of air in motion cere ajjjjroximcoteli/ the same. 



Cause of the difference between the direction of the motion of the up^er 

 and lower currents. — In the preceding inyestigation we hare been occupied 

 simply with numerical relations following directly from observations made 

 without reference to any hypothesis. It will be easy, however, to show 

 -that the results obtained relatively to the different directions of motion 

 of the atmospheric currents are in strict conformity with the reasoning 

 of writers on this subject, from Hadley to Herschel. 



Putting aside altogether the hypothetical ascending and descending 

 currents of air, which were lirst suggested by Hooke, and for which we 

 have no evidence whatever, we owe to Hadley the first indication of the 



If we consider the mean value of {\p) — (p for each stratum to be half that at the 

 upper and lower limits (it shotild probably be more than half), and assume that for 

 the stratum above the cirri to be 32° (?/^ — for the cirri being 29°), we have — 



o 



Soil to scud and cumulus stratum =+ 7'25 



Scud and cumulus to cirro-stratus = 18"65 



Cirro-stratus to cirrus = 26-20 



Cirrus to upper limit of motion = 32 '00 



and give to each equal weight, we find the mean, 



(^)-0=+21°. 



There is no hypothesis that can be proposed, founded on the observed differences, 

 which could affect the final results to any considerable extent, nor the conclusion which 

 is deduced from it. 



t The difference {ip) — (p deduced from the observations at Makerstoun, confirmed 

 approximately by those at Brussels, being supposed to hold at Greenwich and Dublin. 



