564 Transactions. — Physics. 



Anticyclones, Cyclones, and Westerly Wave-pressures have 

 now been described, but none of tbetn have any definite period 

 of occurrence or of duration. All are continually undergoing 

 some process of change, increasing or decreasing both in 

 energy and area. All have their special arrangement of wind- 

 movement, which, though constant to each, is subject to great 

 deflection by the contours of the land and by the action of one 

 system upon another. All have definite but different routes, 

 to which they keep unless deflected from them by a system of 

 different kind and greater energy, and there is no definite 

 period of alternation between high and low pressures. 



The Probable Mechanical Principle on which Atmo- 

 spheric Circulation is based. 



Meteorological investigations have enabled us to define 

 certain systems of pressure, their wind-movements and routes, 

 and from these it may be possible to deduce some further in- 

 formation as to the nature of the principles which guide the 

 movements of our atmosphere. 



Lying adjacent to and on both sides of the Equator are 

 high-pressure systems which form two complete belts round 

 the globe. Their wind-rotations are in opposite directions : 

 those north of the Equator have this movement from right to 

 left, but in those southward of it the rotation is from left to 

 right. In each belt there are many systems, each generally of 

 great extent, but all separate from each other. Their route on 

 both sides of the Equator is from west to east, but none com- 

 plete the circuit of the earth. Between these high-pressure 

 belts lies the Equatorial belt of low pressure, and the latitudinal 

 limits of the high pressures vary from the northern and southern 

 limits of the Equatorial belt to from 30° to 50° of north and 

 south latitude. Within these limits the high pressures are 

 always undergoing changes — expanding, contracting, or merg- 

 ing into oue another. To these changes the Equatorial belt 

 moves sympathetically, and it is therefore irregular in width 

 and sinuous in outline. 



Consideration of these atmospheric conditions discloses a 

 mechanical arrangement in which the anticyclones compos- 

 ing each belt correspond to the movements of a series of 

 circular discs revolving in opposite directions, and having 

 between them a belt upon which they press equally. The 

 revolution of the discs causes the belt to move from right to 

 left — i.e., from east to west; or, if we consider the belt as 

 being the motive power coming from the right, then the bodily 

 movement of the discs must be to the right, or from west 

 to east, and their rotation must correspond with the wind-rota- 

 tion of the anticyclonic systems of each hemisphere, while the 



