698 
is going on. Caves in moderately free communication with the air 
_are dry and (to the feelings) warm in winter, wet or damp and cold 
in summer. And from the general course of this law I do not con- 
sider even your Orenburg caves exempt, since however apparently 
arid the external air at 120° Fahr.! may be, the moisture in it may 
yet be in excess and tending to deposition, when the same air is 
cooled down to many degrees beneath the freezing point. 
“The data wanting in the case of your Orenburg cave are the 
mean temperature of every month in the year of the air, and of ther- 
mometers buried say a foot deep, on two or three points of the sur- . 
face of the hill, which if I understand you right is of gypsum and of 
small elevation. I do not remember the winter temperature of 
Orenburg, but for Catherinenbourg (only 5° north of Orenburg), 
the temperatures are given in Kuppfen’s reports of the returns from 
the Russian magnetic observatories. If anything similar obtains at 
Orenburg I see no difficulty in explaining your phenomenon. Re- 
jecting diurnal fluctuations and confining ourselves to a single sum- 
mer wave of heat propagated downwards alternately with a single 
winter wave of cold, every point at the interior of an insulated hill 
rising above the level plain will be invaded by these waves in suc- 
cession (converging towards the centre in the form of shells similar 
to the external surface), at times which will deviate further from 
mid-winter and mid-summer the deeper the point is in the interior, 
so that at certain depths in the interior, the cold-wave will arrive at 
mid-summer and the heat-wave in mid-winter. A cave (if not very ~ 
wide-mouthed and very airy) penetrating to such a point will have 
its temperature determined by that of the solid rock which forms 
its walls, and will of course be so alternately heated and cooled. 
As the south side of the hill is swnned and the north not, the sum- 
mer wave will be more intense on that side and the wimter less so; 
and thus though the form of the wave will still generally correspond 
with that of the hill, their antensity will vary at different points of each 
wave-surface. The analogy of waves is not strictly that of the pro- 
gress of heat in solids, but nearly enough so for my present purpose. 
“The mean temperature for the three winter months, December, 
January, February, and the three summer months, June, July, Au- 
gust, for the years 1836, 7, 8, and the mean of the year, are for 
Catherinenbourg as follows :— 
Winter. Summer. Annual Mean. 
18386. — 10°93 R. + 11°90 R. PIS e202) a 
1837. — 12°90 + 12°93 + 0°30 
1838. — 12°37 + 12°:37 + 0°°60 
Mean.| — 12°07 R. + 12°40 R. + 0-70 R. 
+ 4°83 Fahr. | + 59°9 Fahr. | + 33°57 Fabr. 
