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Show 136 DEGREES OF HEAT. days are short, and the few hours of mid-day sun would only rouse the energies of vegetation ~or a little, to be destroyed by the rigour of the long ~1ght. But though the light and heat are thus, at those ttmes, in those nlaces, excluded from contact with the earth, and-action upon its vegetable productions, they are not lost. The white surface sends them upwards to warm the air; and as there is little evaporation there, and little vapour in the sky to absorb the heat, the atmosphere maintains a far more comfortable temperature than one would be led to suppose. Thus, in every place, and at every season, there is something in nature to compensate man for what the inhabitants of other countries regard as his privations. Heat is still more wonderful than even light, wonderful as that is, and abundant as are the information and the pleasure which we derive from it. Like light, we never can find heat alone ; for as light is only perceived when something lightens or is lightened ; so we become conscious of the existence of h~at only when something heats or is heated. Thus, as we never can by any process in nature, or any experiment that we can perform artificially, obtain any knowledge either of light or of heat as a distinct substance, or even as a material and measureable part of any substance, we cannot know any thing further of either than as a property of those substances in which we perceive its effects. To speak of the properties either of light or heat is an absurdity, because we know light and heat themselves only as properties ; and therefore all their countless variations are variations only in degree ; and as no property can be the measure of another property in the same way that one weight is the measure of other weights, or one length the measure of other ]engths, there is no standard to which we can bring either light or heat, except we make some degree of each which we find constant, as displayed in some MOONLJGHT. 137 substance, the measure of the other's degrees. The variations of light are so very delicate in themselves, and they are so much confused by the variations of colour, that it is scarcely possible to obtain any contrivance by which light can be made the measure even of itself. Various instruments called photometers, that is, "'light measures," have been invented by ingenious men ; but the majority, if not the whole of these, are affected by, and therefore measure heat, and not light ; and thus they are, in truth, thermometers, or heat measures of more nice construction and greater sensibility than the common ones. It is, indeed, exceedingly difficult even to contemplate light without having the notion of heat along with it ; and, indeed, we have not much knowledge of especially great degrees of heat, without light along with it. In poetical language it is not uncommon to speak of "the wan cold moon," and "the cold moonbeams ;" and there is truth as well as poetry in those expressions. It has been mentioned that the red rays of the sun penetrate the most readily into the substances on which they fall ; and the greatest heat, which is at the red end of the spectrum, penetrates still more readily than the red rays. ~ow, our moonlight really comes from the sun, and 1s reflected to us from the surface of the moon, just as we can throw light into a dark room by a mirror or by whitewashing fl wall opposite the door o~ which light can fall. Now the heat of the sun's !ight, and also the greater part of the red rays, enter mto and are absorbed by the moon ; and thus moonlight wants the golden brightness of the direct rays ~f the sun, a~d is. in ?onsequence silvery, and has a btt}e of a blmsh hnt m it. This "soft ~oo:nlight,'~ not _only delightfully varies the J?Onth~ wt~h Its waxmg, Its fulness, its waning, and Its extmctwn, anrl not only gives us landscapes of new and softened tone, which it would be alto- M2 |