to the Antarctic Regions. 191 



menon; as well as for ascertaining, also apart from those influences, 

 the existence or non-existence of that difference between the diurnal 

 and nocturnal maxima and minima, which has been proved to exist 

 in some localities, and surmised to be general*. 



Connected with the equatorial barometric depression, and the 

 ascensional current of heated air which produces that depression, is 

 a phenomenon which may serve to elucidate the mechanism of this 

 current in its origin, as well as to illustrate the mode in which as- 

 cending currents occasionally produce rain. It cannot be supposed 

 that the whole body of the equatorial atmosphere rises en masse, or 

 with any regularity or steadiness. Such a movement would be out 

 of analogy with what we know of the movements of fluids in general. 

 Its tendency to rise is general, but this tendency is diverted by a 

 thousand local influences, and concentred on particular points, where 

 it results in ascending columns and sheets, between which wind- 

 flaws, capricious in their direction and intensity, and often amount- 

 ing to sharp squalls, mark out the course of their feeders and of the 

 indraft of cooler air from a distance to supply their void. Now the 

 existence of such ascending columns is rendered frequently visible 

 in a very unequivocal way, by vast piled-up masses of cloud of that 

 peculiar form which has been called cumulostratus, the bottom being 

 flat and ill-defined, the u])per parts towering to an immense height, 

 and ragged with great protuberances. From the bases of these 

 great cumular piles are almost constantly seen to descend those vio- 

 lent showers so common in the calm latitudes. 



It would be interesting on many accounts to obtain measures, even 

 if somewhat vague, of the altitudes at which the bases of these clouds 

 rest, as well as of the height of their summits, and to measure the 

 temperature of the rain which falls from them at successive periods, 

 as they pass over the ship, so as to ascertain whether the rain which 

 falls along their axis be not colder (from coming at least in part 

 from a greater elevation) than that from their skirts. The vapour 

 plane, in such circumstances being nearly or precisely uniform over 

 vast tracts of sea, the altitude of the base of such cloud vertically 

 overhead may be considered the same as that of any other favour- 

 ably situated for measure. In fact the determination of the mean 

 height of the vapour plane at and near the equator is one of high me- 

 teorological import, and is connected by no circuitous steps with all 

 the most interesting questions regarding the distribution of aqueous 

 vapour over the globe and the irrigation of the continents. 



5. Distribution of Temperature in the Sea and Land. 



Connected as this subject is with meteorology, it requires in some 

 points of view to be considered apart. As the currents in the atmo- 

 sphere are produced by the diff'erence of temperature in its polar and 

 equatorial regions, so it may be contended are those of the ocean by 

 diff'erences of temperature due to the same geographical causes. Such 



* See Reports of Met. Com. S. A. Phil. Soc. above referred to. 



