702 
reasoning be ice or rock. It is enough, that its surface, during the 
whole or great part of the year, should be covered with ice to bring 
down the mean annual temperature of its interior materially below 
the temperature due to its elevation, and which it would have were 
it not so covered, Conceive now a mountain whose summit is in 
this predicament, viz. constantly maintained at a mean temperature 
below that due to its elevation. This intense cold will not break off 
at the level of the line of perpetual snow, which is determined by 
the mean temperature of the atmosphere due to elevation, but will 
be propagated downwards in the interior of its mass. Hence, if at 
a short distance below the line of perpetual snow, where the mean 
diurnal temperature of the exposed part, taken at a few feet or a few 
yards deep in the soil or rock, is a little above freezing, we drive an 
adit, or take advantage of a natural fissure to obtain the internal 
temperature at a much greater depth from the surface ; we ought to 
find it below 32°, and ice ought constantly to form in such cavities. 
But even when the summit of a hill is not covered with ice, and 
when therefore this particular principle does not apply, it is easy to 
see, on the same general grounds, that something of the same kind 
may obtain. It is obvious, that whenever a change of temperature on 
the surface of a solid takes place, a wave of heat or cold, as the case 
may be, will be propagated through its substance ; and if the changes 
be regularly periodic, the waves will be also. Moreover it is clear 
that the longer the periods of the external fluctuations are supposed, 
the greater will be the interval of the waves, so as to make the 
time taken for the propagated heat to run over them precisely equal 
to the period of fluctuation. Now the rapidity with which succes- 
sive waves of heat and cold destroy each other, is inversely as the 
intervals, and thus the fluctuations of temperature depending on 
long periods of external change will be propagated to greater depths 
than those arising from shorter periods, nearly in the ratio of the 
lengths of the periods. Thus the depths at which the annual fluc- 
tuations of temperature cease to be sensible, will be between 300 and 
400 times greater than those at which the diurnal ones are neutral- 
ized. Now it may happen, from the slowness of propagation through 
so considerable a depth, that the winter wave of cold (consisting of 
many diurnal waves of alternate, greater and less intensity) may not 
travel down to the adit or cavern till the hottest period of the next 
summer, or of many summers; in short, that if at any given time 
the interior of the mountain were sounded by thermometers down its 
whole axis, these instruments would exhibit alternate deviations -+ 
and — from the mean temperature of the air. 
A paper “On Rock-Basins in the Bed of the Toombuddra, South- 
ern India (lat. 15° to 16° N.),” by Lieut. Newbold of the Madras 
Army, was then read. 
Rock-basins abound in the beds of many rivers in southern India, 
particularly where rapids and falls are of frequent occurrence ; but 
in none are they more numerous and better exhibited in their va- 
