ACADEMY OF SCIENCES. 81 
wall beside the fall and as near the sheet as it was possible to breathe or to 
stand. If any of the pellets occurred there, I could not prove it. I could 
not feel them, and the water so blinded us that nothing could be seen 
distinctly. On returning, we kept on the rocks, and noticed none of the ice- 
pellets there. I had left my thermometer behind, and had no means of 
testing the temperature of this freezing blast. 
At Leidig’s Hotel, which is one and three-eighths mile distant and about a 
thousand feet lower, my thermometer stood at about 52° Fahr. at 6 a.m.; 
78140 at 2.30 p.m.; 79° at 3.15. p.m.; 58° at 9 p.m., and 50° at 6 the next 
morning. I had no wet-buib to determine the dryness, but that the air was 
very dry was shown by the rapidity with which our saturated clothes dried. 
When this fall was visited by the State Geological Survey in June, 1863, 
the idea was suggested that we examine the temperature of the water above 
and below the fall, to see if any actual heating of the water occurred as a 
result of its concussion after falling from so vast a height. The dryness of 
the air was then so great that I was convinced that evaporation would coun- 
terbalance or at least vitiate any results that might be theoretically based on 
the mechanical equivalent of heat, so the experiment (which would have cost 
much labor and time) was not tried. And on seeing this new phenomenon, 
the hypothesis which immediately suggested itself to me as an explanation 
was that it was due to evaporation. That the fall is fed by melting snow, 
much of which still’ lies near its top; that the great volume of ice-cold water 
chills the adjacent air to near 32 degrees; that the air-current thus cooled, as 
it is drawn into and along with the immense descending mass, is a very dry 
current, and that its rapid saturation by this evaporation of a portion of the 
spray is sufficiently chilling to freeze drops of water up to a certain diameter. 
Had the ice-pellets been portions of the ice-cone torn off from its edge and 
hurled outward with its spray, we would not expect such an uniformity of size 
as I observed. 
Professor John LeConte, on my describing the phenomenon to him to-day, 
has suggested another hypothesis, more plausible, perhaps, than mine. Itis 
that the air carried down and cooled by the water is somewhat condensed at. 
the base of the fall, and that by its expansion asit gets away from the pressure, 
sufficient cold is produced to freeze the drops. 
Whatever may be the explanation, of the fact there is no mistake. 
T. J. Lowry read the following paper: 
Hydrographic Surveying. 
BY T. J. LOWRY, U. S. COAST SURVEY. 
Hydrographic surveys of bays, iakes, rivers, gulfs and the parts of oceans 
adjacent to coasts, are indispensable requisites to a safe navigation, and hence 
successful international commerce. Being of national importance, they are 
therefore national undertakings—and the Government Coast Surveys and 
Proc, Cau. AcapD. Scr., Vou. VI.—6. 
