I shall first inquire, says Dr. Rennie, what are the effects of climate on healthy constitutions, as respects heat, cold, moisture, and vicissitudes; including also the diurnal and annual revolutions. Cold applied to the body acts as a direct sedative. It diminishes the nervous sensibility, represses the activity of the circulation, detracts from the sum of the animal heat, and thereby diminishes stimulation. In the cessation of excitement and sensibility that ensues, the whole vital actions are moderated, existing irritation is soothed; and in the same manner as sleep recruits the wasted powers, so does cold restore and invigorate the nerves when overstimulated, and in fact promotes the tone and vigour of the whole body; when again a warmer atmosphere succeeds a colder, the animal heat increases in its sum, the surface of the body is re-excited, nervous sensibility returns, and a reaction of the circulation takes place; so that the blood diffuses itself in greater abundance towards the remote and superficial parts of the body, and the secretions are also promoted. Alternations of cold and heat therefore in healthy constitutions within certain limits, are salutary; promoting, on the one hand, the vigour and tone of the body; on the other, the due activity and excitement of the various functions. The temperature occasioned by day and night, and also those more progressive and slow alternations of heat and cold, on the large scale, attending the annual revolution of the seasons, are a natural provision admirably adapted to effect these objects as described; constituted as our bodies are, such a constant and regular succession of heat and cold is just such as the necessities of the human frame require. The alternations of day and night, of winter and summer, are far from being merely incidental and unimportant circumstances in the general adaptation of the earth to man's constitutional wants; neither do they bear reference solely to the productions of the earth for his use. They exert a continual and direct influence on his constitution, calculated to aid the vigorous and healthy performance of the various functions of the body each in its due degree and order, and they conduce mainly to the perfection and longevity of the species. Let us therefore trace the effects of these changes on the human body. During the winter, the prevailing cold acts as a universal sedative and tonic, soothing the nervous excitement and sensibility, allaying the activity of the circulation, moderating the functions of the skin, and diminishing the various secretions. As the Spring opens, the sun gains daily in influence, generating a gradually increasing atmospheric warmth. The body therefore becomes subject from this heat to a reactive effect, during which the nervous sensibility and circulation are gradually re-excited, the blood is more equally diffused towards the surface and extremities of the body, and the secretion by the skin is increased.