﻿and Fluctuations. 327 



the cause, which is the effect ; and when, as in this instance, two 

 classes of phenomena (atmospheric movements and variations in 

 barometric pressure) are so closely connected, and stand so ma- 

 nifestly towards each other in the nearest relationship, in the 

 absence of further information we have no means of deciding 

 whether the changes in barometric pressure cause the atmo- 

 spheric movements, whether the atmospheric movements cause 

 the changes in the barometric pressure, or whether some third 

 and extraneous agency does not cause both the movements of 

 the atmosphere and the fluctuations in its pressure. These con- 

 clusions seem, at first sight, equally to merit our acceptance ; 

 and notwithstanding the prescriptive right to our belief claimed 

 for the first mentioned (by reason of old traditions and scientific 

 conservatism), an examination into the grounds of this belief is 

 at least necessary before we can yield an entire and rational 

 assent. 



It is therefore the causes of barometric fluctuations that I pro- 

 pose to inquire into; and since in meteorology, more almost than 

 in any other science, incorrect statements have been frequently put 

 forward, resting on some plausible theory, the fallacy of which 

 it is difficult to point out, I propose submitting each step in our 

 inquiry to the test of experimental and geographical fact : if it 

 cannot stand this, it is good for nothing ; if it can, we may con- 

 sider it a certain, however slight advance towards establishing 

 our conclusions on a satisfactory basis. 



At the very outset it is necessary to call attention to the fact 

 that the barometer does not measure weight : the name unfortu- 

 nately implies that it does; but we all know (if we can only bear 

 it in mind) that it does not. What the barometer measures is 

 elastic force ; with weight or density it is concerned only so far 

 as they are functions of elastic force. The distinction between 

 weight and elastic force cannot be too strongly insisted on ; and 

 the want of observing it has led to great confusion. It has thus 

 been almost universally assumed that heat lowers the barometric 

 pressure, and that cold increases it : I find this repeatedly stated 

 in the most absolute manner by meteorologists of high repute; 

 and yet, as an abstract fact, it is distinctly contrary to the truth. 

 Heat, while it tends to increase the volume, does certainly tend 

 to decrease the density ; but while exercising this tendency, it 

 actually increases the elastic force of the air. Cold (under- 

 standing by cold merely a diminution of heat) works in exactly 

 the opposite way ; it tends to decrease the volume and to in- 

 crease the density, but does meanwhile lower the elastic force. 

 If, then, the increase or decrease of volume does not keep pace 

 with the increase or decrease of temperature, the barometric 

 pressure is raised by heat and lowered by cold. This is easily 



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