﻿THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MAY 1871. 



XLIV. On Barometric Differences and Fluctuations. By John 

 Knox Laughton, M.A., F.R.A.S., F.R.G.S., Mathematical 

 and Naval Instructor at the Royal Naval College, Portsmouth*. 



Part I. The Influence of Temperature and Humidity on Baro- 

 metric Pressure. 



TILL within the last few years meteorologists invariably 

 commenced any inquiry into the movements of the air by 

 the statement, however differently worded, that all movement is 

 caused by the unequal distribution of heat at the surface of the 

 earth. This unequal distribution of heat was supposed to act by 

 the unequal expansion and rarefaction of contiguous masses of 

 air, by the consequent difference of their density, and the dis- 

 turbance of equilibrium. From a hydrostatic point of view such 

 a statement seemed theoretically perfect ; and it was therefore 

 laid down with the most absolute confidence as a meteorological 

 axiom. Later and more exact observations, extending over a 

 wider geographical field, have established its incorrectness, and 

 have shown very conclusively that the mere expansion of air by 

 heat, and its increase of density by comparative cold, do not 

 account for the anemological phenomena which really take place, 

 and in many instances seem to be in direct opposition to them. 

 No one can doubt that heat, acting on the air, does produce very 

 important results; but it is now equally impossible to doubt 

 that, whatever its effects may be, they are so mixed up with the 

 effects produced by other agencies (such, for instance, as evapo- 

 ration or condensation), that no class of phenomena can be attri- 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 41. No. 274. May 1871. Z 



