In winter, the external
columns of air are as much heavier than the columns in the fissures as
they are lighter in summer; and so cold currents of air blow from the
cave into the fissures, though such currents are not of course colder
than the external air. Thus the mean temperature of the cave is much
lower than that of the rock in which it occurs; for the temperature of
the currents varies from the mean temperature of the rock to the winter
temperature of the external atmosphere.
The descending columns of warmer air, in summer, must to some extent
raise the temperature of the fissures above that which they would
otherwise possess, that is, above the mean temperature of the place; but
that may be considered to be counteracted by the corresponding lowering
of the temperature of the fissures by the introduction of cold air from
the cave in winter. By a similar reasoning, it will be seen that for
some time after the spring change of direction in the currents takes
place, the temperature of the cave will be less than would have been
expected from a calculation founded on the true mean temperature of the
rock through which the fissures pass.
Pages:
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465