﻿332 Mr. J. K. Laughton on Barometric Differences 



fallacy which has escaped detection, until a rigorous application 

 of the test I have proposed (that of geographical comparison) 

 compels us to recognize its existence. It appears to me to lie 

 in the unauthorized assumption that any increase or decrease of 

 temperature or humidity which affects the lower strata of the 

 atmosphere extends upwards to its very furthest limits. This 

 assumption may probably, to a great extent, be attributed to the 

 incorrect way of speaking now so general, and which indirectly 

 teaches us to think that a mass of air being made specifically 

 lighter than it was, whether by expansion from heat or by ex- 

 pansion consequent on the intermixture of aqueous vapour, has 

 a tendency to rise, and does rise. We know that this is not the 

 case : we know that a piece of cork at the bottom of a basin has 

 no tendency to rise, and does not rise ; but if water is in the 

 basin, the cork is raised by the pressure of the water underneath 

 it. But popular usage says that the cork rises ; and it would now 

 almost savour of pedantry to insist on the more correct expres- 

 sion. Still we may and must insist on a clear understanding of 

 the sense in which the word is used : custom may modify the 

 laws of grammar ; but the laws of gravitation are beyond its 

 power. 



It is thus, however, that a mass of air heated or rendered 

 more humid has been supposed to rise, as though by virtue of 

 some quality imparted to it which causes the earth to repel it ; 

 yielding to this, it rises and continues to rise to the very top of 

 the atmosphere, where its power of rising suddenly ceases ; the 

 repellant force is changed into one of attraction ; and the mass 

 of heated air, finding itself above the air immediately near it, 

 flows over it, and increases the height and therefore the weight 

 of the adjoining atmospheric column. Put into plain and un- 

 mistakable language, this statement would shock the common 

 sense of any one with the smallest pretension to scientific know- 

 ledge : it is none the less the statement which is repeated in 

 almost every work on popular meteorology, as well as in many 

 of a much higher order of merit. We know that it is not true ; 

 for we know that a mass of air, whether hot or cold, moist or dry, 

 has absolutely no tendency to rise, but, on the contrary, is drawn 

 down by all the force of gravity. We know that it is not true ; 

 for countless observations in almost every part of the earth prove 

 to us that the height to which the heat or humidity of the air 

 extends is very limited. An explanation of the way in which 

 strata of hot and cold, of moist and dry air are intermixed is 

 one of the most curious and interesting problems in meteorology ; 

 but that they are intermixed, that a hot stratum, or a wet stra- 

 tum, or any other stratum is found frequently lying between two 

 strata of a very opposite condition, forming a sort of atmospheric 



