﻿326 Mr. J. K. Laugh ton on Barometric Differences 



buted solely to this property of expansion and contraction. 

 That, according to this theory, wind blows, necessarily and in- 

 variably, from the place of less towards the place of greater heat, 

 is contrary both to our geographical knowledge and to our com- 

 mon experience; and whilst loth to admit in plain language 

 that the effect due to heat, in this manner, has been overstated, 

 the leading meteorologists of the present day have virtually sub- 

 stituted for the old axiom a modification of it which may be 

 shortly stated thus : — 



The movements of the air are caused by differences of baro- 

 metric pressure : air necessarily moves from the place of high 

 pressure to the place of low. 



And on this axiom, the system of all modern writers, more 

 especially of Mr. Buchan of Edinburgh, and Professor Mohn of 

 Christiania, is explicitly founded. 



Before, however, we can unreservedly accept such an axiom, 

 we must have a clear idea of its exact meaning; before we can 

 admit that differences of barometric pressure are the causes of 

 the atmospheric movements, we must have some insight into the 

 cause of these barometric differences themselves; and it is this 

 point which meteorologists, without exception, have slurred over : 

 however lucid their writings may be in other respects, here their 

 clearness fails them ; and frequently the only inference that can 

 be drawn is, that barometric differences are a first cause, beyond 

 which it is not necessary to inquire. This, of course, is not 

 what they mean ; but, admitting that it is a question of great 

 difficulty, they have rather avoided than grappled with it. With 

 the various explanations, or partial explanations, that have been 

 offered, no one can feel satisfied; and yet, till some satisfactory 

 explanation is arrived at, meteorological research tends.. to become 

 merely a dry repetition of the observations of phenomena which 

 we can dimly see are connected in some way with each other, 

 without being able in the least to trace that connexion to its 

 source. Until the phenomena of barometric fluctuations and 

 differences are understood, we are not at liberty to assume that 

 they are the causes of atmospheric movements. We know that 

 a barometric difference between two adjacent masses of air will, 

 under certain conditions, establish a movement between the two : 

 experimentally, we know that air will rush, even with great force, 

 out of a vessel in which it has been compressed ; but we also 

 know that under certain conditions a movement of air may raise 

 or depress the barometer — a fact that we may readily illustrate 

 by driving a current of air, by a force-pump, into a glass vessel 

 which contains an aneroid. 



When, of two observed phenomena, either one can produce the 

 other, very great caution is necessary before we decide which is 



