The state of the atmosphere in regard to motion or rest modifies to a great extent the effects of a given temperature upon the body. Refrigeration by simple contact increases in amount with the rate of motion of the air. The same law holds good in regard to evaporation, and indeed this process always complicates the results proceeding from simple contact. The cause of refrigeration in this case is consequently double. It is easy, therefore, to imagine how powerful a cause of cooling a cold wind must be. But observation can alone give any ade quate idea of the extent of its influence in this respect. Mr. Fisher, one of the surgeons in the expedition under the command of Sir Edward Parry to the Polar Seas, has given us an account of its extraordinary effects. In the frozen regions around the arctic circle, the hardy voyagers under Capt. Sir E. Parry found that they could stand a cold adequate to freeze mercury when the air was perfectly calm, much more easily than a temperature nearly 50° F. higher when it blew. The air in motion in this case, therefore, produced a sensation of cold that was equal to such a depression of temperature as is indicated by a fall of 50° of the scale of F.—a most prodigious difference.
Sudden transitions of temperature also exert a great influence independently of any limits ; in the first place, because the intenseness of the sensation of cold or of heat is in propor tion to the suddenness of the abstraction, or of the communication of heat ; and again, be cause the faculty of adaptation to different degrees of external temperature is not acquired all at once, but is only attained in a certain lapse of time, and by gradual modifications in the constitution. We therefore see that those countries of which the temperature is very high in the day, but very low in the night, are subject to diseases that seem to belong more peculiarly to cold and moist latitudes, or to marshy lands where malaria prevails. But the transition from hot to cold is not limited to the suddenness of the thermal de pression ; it extends to the refrigeration by the action of the wind. This is another among the many reasons why in the latitudes of Eng land, France, &e. spring is a more dangerous season than autumn. There are, however, cer tain cases of sudden transition that are useful and salutary, as for instanee,when the heat of the body is excessive, and is doing mischief, whe ther it be induced by an elevated external temperature, or proceeds from the violent and involuntary action of our organs. Then re frigeration even of the most sudden kind, pro vided it be restrained within proper limits, becomes beneficial. It is thus that the affusion of cold water produces such excellent effects in cases of extreme excitement, and where the temperature is really above the natural standard. This process is even to be regarded as one of the mast brilliant tri umphs of modern medicine. It is much to
be regretted that recourse is not had to it more frequently. It is evident that the proper time for the use of this powerful means is that in which congestion has not yet passed into ob stinate engorzernent, that is to say, in the beginning of the disease, in which by allaying excitement congestion is diminished. The favourable moment for using the cold affusion is that in which the skin is hot and dry, which is also the period of the highest excitation. The experiments upon the effects of baths, quoted above, tend also to show the propriety of the practice ; in citing these, we mentioned that the diminution of temperature produced in the body lasted for hours, and that the reaction consequent upon the use of the bath did not carry the temperature higher than the pitch it possessed at starting. It is obvious that the effects of the cold affusion are to be derived from the principles previously established ; since we have referred the production of heat to two general conditions of the economy, one of which is the state of the nervous sys tem. Now the allusion of cold water acts directly upon this system. There is another powerful method of tempering animal heat, which flows from the other general condition, upon which the production of heat depends, viz. the state of the blood. We have seen above that the respective proportions of the serous mass of the blood and of its red glo bules exert an important influence; that in the class of vertebrate animals which produce smaller quantities of heat, the proportion of the serum was in the inverse ratio of the faculty of calorification. NVhence it follows, that in eases of excessive heat of body, to reduce the quantity of red globules would prove an effectual mode of reducing the tem perature. Now this is precisely what is done by bloodletting. The effect, however, in this way is not instantaneous. The first influence of bloodletting is simply to lessen the quan tity of the blood, and this is the extent to which ideas of the influence of the abstraction of blood are generally confined. There is, however, a consecutive influence, which is at the least as important, and which proves much more lasting. As the person who has been let blood confines himself at the same time to low diet, and principally to liquids, it is obvious that the blood is recruited in its quantity principally by additions of watery particles, without any notable or even sensible additinn of globules. The blood is therefore altered essentially in its constitution ; the proportion of its component fluid and solid elements is changed, and this in direct proportion to the extent and frequency of the vene,ections. The consequence of this is a diminution of tempe rature, unless other causes oppose such an ef fect.