20 C. PIAZZI SMYTH ON 



higher still, they appear so blindingly white, they come into contact with "the 

 Tenerifte air above the clouds " (i.e., the clouds of the lower N. East wind), 

 which air is dry to the last degree ; dry even to a depression of from 25 to 30 

 degrees Fahr. of the wet, below the dry, bulb Thermometer. Now this is a 

 state of things which must evaporate the outside of the watery cloud-molecule; 

 and if there be, as I believe has long since been generally held, a hollow or air 

 centre to it, must leave the watery coating portion only as a thin shell sur- 

 rounding such air particle. In which case such shell will reflect from a second 

 surface almost as large as the first, and with a very minimum of absorption by 

 fluid material. 



Such thinning indeed of the vesicle, as pointed out lately by Professor Alex. 

 Herschel, must not be carried too far ; or, like the black centre of Newton's 

 rings, there will supervene an incapacity to reflect any light ; immediately after 

 which, the vesicle must burst, and cease to be a visible existency. As this 

 termination of the life of cloud molecules — which are seen by travellers who 

 ascend the peak, to be continually rising into the upper air, but never getting 

 beyond a certain level therein,* — must take place more rapidly the drier the 

 medium, it results that the whole cloud seen in the distance will have a harder, 

 better defined ontline, on the side where the air is dry, than that where it is 

 moist; or according to Nature's general law, above, rather than below, the level 

 of the N. East wind's clouds. 



Now the night clouds of April 8, 1882, which belonged to such N. East 

 weather, showed well-compacted, almost case-hardened surfaces below, or when 

 looked at from below, which was also at that time, the direction from which 

 their chief illumination came, viz., the city gas-lights. And if their lower 

 surfaces were then so compact, and could then reflect so powerfully, as they 

 undoubtedly did that night, it must have been because there was at that time 

 a stratum of air below them, as remarkably dry as the classic stratum which is 



* This feature is well set forth by Dr Marcet, F.K.S., in his recent hook Southern and Swiss 

 Health Resorts, p. 262 ; except that he speaks of the return S. West current as being immediately 

 above the cloud, in place of, as it is throughout all the summer season, separated from it by a thickness 

 of full 5000 feet of a gradually decreasing strength of N. East wind, the same in direction as what prevails 

 both below, and in, the cloud level, but differing hygrometrically therefrom exceedingly, in being extra- 

 ordinarily dry. This important physical peculiarity is afterwards, however, fully acknowledged by Dv 

 Mabobt at pp. 296 to 306 of his useful book. For he there sets forth in a fuller and more serious 

 manner that the N. East cloud level is never so low as 1200 feet, but nearer to 3000 feet high; and 

 by its shade moderates both the temperature, the radiation and the moisture of the country below it. 

 l!ut at his Guajara station, 7090 feet high and therefore altogether above that N. East cloud level, he 

 found there was still a prevailing tendency of the wind to blow from the North East, but accompanied 

 by a terrific dryness, amounting on one occasion to 30° - 5 depression of the wet, below the dry, bulb 

 thermometer. Even on the higher central Peak, at 10,700 feet elevation, he says that " the -S. West 

 current, bringing hack moisture from between the tropics," was only beginning to be felt ; and the 

 traveller would have to ascend several thousand feet higher still, if he could, to reach the full force and 

 volume of that important stratum, at that season. All which exactly agrees with my own experience, 

 described in Tenenffe, an Astronomer's Experiment, in 1856. 



