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.
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.
As the season passes on from the Equinox towards the winter solstice, the heat of the sun daily diminishes, and the cold gains a daily preponderance. The sedative effect on the body goes on progressively increasing, being less and less counteracted by any genial influence from the solar heat at mid-day; whence the gloom and depression so universally experienced by the nervous in November and December, which is more and more felt till the shortest day. So soon as the minimum of solar influence and maximum of sedative effect on the body has passed over, the sun gradually acquires more of meridian influence, and a daily increasing ascendancy over the prevalent cold. The human constitution at the same time is subject to a proportionate reactive disposition; which reaction is felt most at noon, and it daily becomes more and more apparent till the vernal equinox, when we have the difference betwixt the meridian and midnight temperature again at a maximum. We have daily a powerful sedative effect in the morning, a powerful meridian reaction, which again subsides into a sedative condition on the access of the evening. This daily effect on the constitution is exactly similar to that at the autumnal equinox, only it occurs under different circumstances. In autumn it is connected with departing heat and progressively increasing cold; in Spring it is connected with progressively diminishing cold and advancing heat. After the vernal equinox, the difference in the meridian and midnight temperature gradually diminishes; the daily sedative effect at morning and evening becomes less and less apparent as general atmospheric warmth prevails, till towards the summer solstice, the general effect on the constitution is stimulation and excitement by atmospheric heat.