﻿Chemistry and Physics. 393 



but at this moment our knowledge of the principal oceanic circu- 

 lation, and of its annual variability, is very meagre. In the course 

 of a few years we may expect a considerable accession to our 

 knowledge, when the Meteorological Office shall have completed 

 a work but just begun — viz., the analysis of ships' logs for some 

 sixty years, for the purpose of laying down in charts the oceanic 

 currents. 



With regard to the great* atmospheric currents even the general 

 scheme is not yet known. Nearly thirty years ago Professor 

 James Thomson gave before this Association at Dublin an im- 

 portant suggestion on this point. As it has been passed over in 

 complete silence ever since, the present seems to be a good oppor- 

 tunity of redirecting attention to it. 



According to Halley's theory of atmospheric circulation, the 

 hot air rises at the equator and floats north and south in two 

 grand upper currents, and it then acquires a westward motion 

 relatively to the earth's surface, in consequence of the earth's 

 rotation. Also the cold air at the pole sinks and spreads out over 

 the earth's surface in a southerly current, at first with a westerly 

 tendency, because the air conies from the higher regions of the 

 atmosphere, and afterwards due south, and then easterly, when it 

 is left behind by the earth in its rotation. 



Now Professor Thomson remarks that this theory disagrees 

 with fact in as far as that in our latitudes, the winds, though 

 westerly, have a poleward tendency, instead of the reverse. 



In the face of this discrepancy he maintains that "the great 

 circulation already described does actually occur, but occurs sub- 

 ject to this modification, that a thin stratum of air on the surface 

 of the earth in the latitudes higher than 30° — a stratum in which 

 the inhabitants of those latitudes have their existence, and of 

 which the movements constitute the observed winds of those 

 latitudes — being by friction and impulses on the surface of the 

 earth, retarded with reference to the rapid whirl or vortex-motion 

 from west to east of the great mass of air above it, tends to flow 

 toward the pole, and actually does so flow to supply the partial 

 void in the central parts of that vortex due to the centrifugal 

 force of its revolution. Thus it appears that in the temperate 

 latitudes there are three currents at different heights — that the 

 uppermost moves toward the pole, and is part of a grand primary 

 circulation between equatorial and polar regions; that the lower- 

 most moves also toward the pole, but is only a thin stratum 

 forming part of a secondary circulation ; that the middle current 

 moves from the pole, and constitutes the return current for both 

 the preceding ; and that all these three currents have a prevail- 

 ing motion from west to east (Brit. Assoc. Report, Dublin, 1 857, 

 pp. 38, 39). 



Such, then, appears to be our present state of ignorance of these 

 great terrestrial actions, and any speculation as to the precise 

 effect of changes in the annual distribution of the sun's heat must 



