TRANSACTIONS OF SECTION A. 531 



3rd. The vertical motion in the atmosphere produced by the heat of the sun's 

 rays expanding the lower air. 



The first of these is universally recognised as the initial cause of all clouds, pro- 

 ducing the cumulus, the firstborn or primary cloud. Its origin is so well under- 

 stood that it is unnecessary to enter on the subject further than is required to make 

 the remarks that follow more intelligible or complete. 



Water, or moisture on the earth's surface, is evaporated by the sun's rays, and 

 enters the adjoining atmosphere. Vapour being specifically much lighter than the 

 air rises more or less rapidly, according to the rate of evaporation. In this climate 

 it has not to ascend to any great elevation before the upper portion arrives in a 

 colder and more attenuated condition of the atmosphere, where it becomes con- 

 densed, but where the lower and flatter portion of the condensed vapour with the 

 uncondensed invisible vapour immediately beneath terminates, it is not possible 

 to say, as a certain amount of the invisible vapour often extends to the earth's 

 surface. All that can with certainty be asserted is that a large mass or bed of 

 air charged with vapour must exist immediately beneath the newly formed cloud 

 or it could not have risen, an amount of floating power being necessary to lift the 

 condensed vapour or cumulus into a cold atmosphere, where it becomes visible. Yet 

 when it has been so raised the friction of the surfaces of the minute globules of water 

 of which it is composed render them unable to penetrate or force themselves down- 

 wards through the air below, or but very slowly. It is therefore only in first raising 

 or lifting up the condensed vapour that the floating power of the uncondensed or 

 invisible vapour beneath is required, and not in sustaining it when raised. 



The invisible vapour, however, that has raised the cumulus or crowning head of 

 condensed vapour, will itself become visible should the atmosphere in which it is 

 travelling become reduced in temperature by any of the cooling influences to 

 which it may be exposed in its travels. 



The lower atmosphere being always charged with more or less vapour, the 

 cumulus can only be formed when there is so much vapour generated as to float 

 above condensation height. 



Thus far a calm is presumed to prevail. 



When, however, the atmosphere is in motion its diflTerential horizontal move- 

 ment produces the first important modification in the foim of the cumulus, the 

 height or depth of the lower current varies, and in this climate will generally much 

 more than include the cumulus or condensation height in these latitudes. The 

 friction of the earth from the irregularities of its surface, and the denser state of 

 the lower air, causing it to flow less rapidly than that which is higher and more 

 attenuated, the upper portion of a cloud moves more rapidly than the lower, and 

 the cumulus shears over into a slanting position, and finally assumes the form of 

 the cumulo-stratus, and, however reduced in depth or thickness the cloud may be- 

 come by this flattening and somewhat attenuating process, the cumulus character, 

 though much diminished, is seldom if ever entirely obliterated.' A young cloud is 

 thus distinguishable from a long-travelled one ; indeed, one that has gone but a short 

 distance is detectable. 



In this climate a large well-developed cumulus is seldom formed in the cold 

 season, but they are frequent in the summer. The majority of the clouds of the 

 first stage seen in this country are born in warmer latitudes, and come as travelled 

 cumuli, and show more or less the condition of the cumulo-stratus. They may be 

 enlarged or diminished from below, and also diminished or increased in number, 

 according to the heat or dryness or dampness they meet with during their passage 

 over the surface of the earth or sea, but my object is not to treat of cloud genera- 

 tion but only to suggest reasons for certain leading forms they assume after birth. 



The invisible vapour, however, is subjected to the same changes of form due to 



> Those who have been much on the sea may probably remember occasionally 

 seeing a large flat brown cloud of smoke in the not remote distance, that has been 

 discharged by a previously passing steamer. This came forth from its funnel in great 

 rounded masses far removed from a flat form. The differential action is thus most 

 clearly and simply illustrated in the change that has taken place since its dis- 

 charge. 



H K 2 



