If the cold of winter were to continue unmitigated from year to year, without the genial influence of summer, the human race, as is apparent in polar regions and upland mountainous districts, would degenerate into dwarfishness. If the heat of summer were continually maintained the whole year round, a tendency to degeneracy of the race would be also observed, as we see in tropical latitudes. It is in the medium betwixt these extremes, where a moderate and regular winter cold is succeeded by a mild, genial summer temperature, that the species approaches most to perfection in stature, health, strength, and longevity. In observing also the influence of day and night on the constitution, there is a sedative effect produced in the morning before the sun is up, a reactive tendency promoted towards noon under the solar influence, and again towards evening this reaction is repressed by the sedative effect of the evening cold; and this sedative effect is at its maximum at midnight. Hence those who sit up late feel unusually chilly and depressed towards midnight, partly owing to exhaustion from want of sleep, but chiefly from the total absence of solar influence in the atmospherical temperature. In regular habits this sedative effect is never thoroughly experienced; for before midnight, the constitution, enveloped in warm blankets, has experienced the reaction arising from the accumulation of heat in bed. Whence the common remark, that one hour's sleep before midnight is worth three after that hour, is actually true to a certain extent. By early retirement to rest, the sedative effect on the constitution, to an extent such as to disturb the functions, is escaped. If we connect these two influences, the annual and diurnal successions of cold and heat, in their joint effect, we find, that about, or a little after the summer solstice, the influence of the sun being at its maximum, the nervous sensibility, heat, circulating excitement, and cutaneous secretions of the body, are also at their maximum. The temperature of the day and night differ so little, that the sedative effects of evening and morning are not sufficient to restore the frame by soothing the sensibilities, overexcited and irritable from the previous warmth. Whence the languor and irritability felt in summer, when the heat is long continued, and the nights are spent in restlessness and anxious oppression. Exhaustion and relaxation of the frame are the consequence. As the autumnal equinox verges on, the mornings and evenings get cooler in relation to the mid-day heat; and about the equinox, the difference in the temperature of mid-day and midnight is at its maximum. We have therefore a powerful sedative effect in the morning, which braces and invigorates the body; a powerful reactive effect at mid-day, which rouses and stimulates the actions and sensibilities of the frame; and again towards evening a sedative effect, from the increasing cold reaching its maximum at midnight.