﻿344 Mr. J. K. Laughton on Barometric Differences 



fact between the parallels of 45° and 55° it blows a gale of greater 

 or less severity almost the whole year through. South of the 

 parallel of 60° these winds appear to lose both their force and 

 their regularity ; and inside the antarctic circle the winds are 

 very variable, and by no means excessive in strength. 



It is thus quite impossible to admit that these winds are in 

 any way caused by the low barometer to the southward of them. 

 They do not blow towards it either directly or in a vorticose 

 manner, but preserve their due westerly direction as long as they 

 preserve their distinctive force, and even after they have in a 

 great measure lost it. There is not a particle of evidence in 

 support of any idea of a vorticose movement ; and however the 

 winds may be caused, and, so far as the present question is con- 

 cerned, whether they are rightly named counter-trades or not, 

 we may consider it clearly established that they are not caused 

 by the low barometer of the Antarctic. And, indeed, if that low 

 barometer is itself due to such a centrifugal tendency, it is im- 

 possible that it should cause any inward movement of the air. 

 The spheroidal form which the atmospheric strata have been 

 compelled to assume is the form of equilibrium ; and that once 

 taken, the centrifugal tendency is powerless to cause motion, 

 save only when it ejects, and sometimes with violence, any excess 

 of air which by some meteorological change has been driven 

 southwards. 



The condition of the atmosphere in high southern latitudes is 

 therefore that of a large body of air in a state of comparative 

 rest, past which a large and strong current is ceaselessly rush- 

 ing. According to the nature of fluids under such a condition, 

 the mass in motion tears away particles from the mass at rest, 

 and establishes a constant tendency to rarefaction. This is the 

 property of fluids first set forth by Daniel Bernoulli in his Hy- 

 drodynamica, where he shows by a series of experiments that, if 

 one end of a pipe be fitted as a branch into another somewhat 

 larger and opening freely, water running through the larger pipe 

 will draw water up from a vessel into which the lower end of the 

 smaller pipe has been inserted*. The friction of the water 

 rushing through the main pipe against the end of the column of 

 air in the smaller tears away the air bit by bit ; and as water 

 from the vessel below is forced up into the vacuum so formed, 

 it too is torn off and carried away. If instead of leading into a 

 closed vessel of water the branch lead into a closed vessel contain- 

 ing air, this air may in exactly the same manner be drawn out till 

 its elastic force is exhausted; and in this way, by keeping a con- 

 tinuous stream of mercury running through the main pipe, the 



* Hydrodynamica, sive de viribus et motibus fluidorum comment arii, 17 3$, 

 p. 276. 



