It is found that there is a given depth beneath the surface of the globe at which an equal temperature prevails. The depth increases as we travel south or north from the equator, and corresponds with the shape of the earth's surface, _sinking under the valleys, and rising under the hills_.
224. _Why may we not understand that this internal heat of the earth arises, as has been supposed by many philosophers, from internal combustion?_
Because recent investigations have thrown considerable and satisfactory light upon the subject. It has been ascertained that the internal temperature of the earth _increases_ to a certain depth, _one degree in every fifty feet_. But that below that depth the temperature _begins to decline_, and continues to do so with every increase of depth.
Yes. They both _absorb_ and _radiate_ heat, under varying circumstances. The majestic tree, the meek flower, the unpretending grass, all perform a part in the grand alchemy of nature.
[Verse: "Consider the lilies of the field, how they grow; they toil not, neither do they spin."]
When we gaze upon a rose it is not its beauty alone that should impress us: every moment of that flower's life is devoted to the fulfilment of its part in the grand scheme of the universe. It decomposes the rays of solar light, and sends the red rays only to our eyes. It absorbs or radiates heat, according to the temperature of the ærial mantle that wraps alike the flower and the man. It distills the gaseous vapours, and restores to man the vital air on which he lives. It takes into its own substance, and incorporates with its own frame, the carbon and the hydrogen of which man has no immediate need. It drinks the dew-drop or the rain-drop, and gives forth its sweet odour as a thanksgiving. And when it dies, it preaches eloquently to beauty, pointing to the end that is to come!
226. _How do we know that plants operate upon the solar and atmospheric heat?_
A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.