﻿348 Mr. J. K. Laughton on Barometric Differences 



the world, they certainly cannot be referred to the small patch 

 of low pressure near Iceland. Whatever their cause, it is inde- 

 pendent of this. But by the ordinary laws which regulate the 

 motion of fluids, a current of air, such as that of the west winds 

 which rage in the Atlantic through the winter, must, on striking 

 against a barrier across its path, be thrown back on itself in a 

 circling eddy ; and it seems to me a necessary consequence of 

 the coast-line and mountains of Scotland and Norway that a 

 large eddy should be formed just where we find the circuit round 

 Iceland. It is therefore to the westerly winds of the Atlantic 

 that I refer this circuit ; and it is to the circuit that I refer the 

 low barometer near its centre. 



But if winds circling round any area tend to constitute it an 

 area of low pressure (and I have shown that by the laws of the 

 friction of fluids they must do so), a very remarkable anomaly 

 is observed in each of the large oceanic basins, where, in the 

 centre of the circuit formed by the gradual veering of the trade- 

 winds on one side and of the west winds on the other, there is 

 an area of distinctly high pressure. In the North Atlantic this 

 area has from the earliest times borne the name of the Horse 

 Latitudes ; but it differs in no material point from the similar 

 areas in the other oceans. The point of difference between 

 these and the other areas in which a low pressure results from 

 the circling of the wind suggests itself at once. The areas of 

 high pressure are areas of warm, almost tropical ocean ; and 

 without referring to any hygrometric observations, we know 

 that in these areas calms are frequent, and that during the calms 

 the air at a high temperature must be tending towards satura- 

 tion. Since, then, a great part of the air supplied to these re- 

 gions from the polar or colder side enters with a mean tempera- 

 ture of 50° or 55° F., and containing vapour of an elastic force 

 of *3 of an inch at the most, and remains warming itself, ex- 

 panding itself, and drawing moisture from the ocean at a mean 

 temperature of 65° F. till the elastic force of its vapour is not 

 less than *6 of an inch, we may fairly attribute the high pressure 

 in these areas in a great measure to the increase in the elastic 

 force of the vapour. 



These several instances of barometric variation are in a great 

 measure typical or representative; and considering them as 

 such, I would sum up the analysis of them which I have at- 

 tempted in a statement of the conclusions at which I arrive. 



By reason of the centrifugal tendency caused by the rotation 

 of the earth, the height of the atmospheric column varies with 

 the latitude ; and with it the height of the barometer, if not 

 controlled by other agencies, also varies, changing gradually 

 fr^m about 29 inches near the poles to about 301 inches near 



