Excessive heat with dryness, as under the blasts of the Simoon or the Harmattan of Arabia or Northern Africa (apart from insolation, sunstroke, or heat-stroke), may sometimes parch the body even to a fatal degree. Much more common is the combination of high temperature with humidity. This has a relaxing effect, promoting indolence of temperament and predisposing to disorders of a catarrhal nature, especially of the digestive organs, such as were called fluxes by the older writers. Cold climates are well known to present the greatest number of cases of acute and chronic affections of organs of the respiratory system; warm and hot climates, those of the stomach, liver, spleen, and bowels. But we must recollect what various complications belong to climate. Two important factors, especially, must be kept in view in comparing the causation of diseases in colder and warmer countries--namely, the difference in the articles of food partaken of in each, and the external sources of enthetic disorders; _e.g._ endemic and epidemic fevers, etc. With humidity must be considered variations in atmospheric pressure. Physicists have long known that while watery vapor, by itself, is heavier than air which is perfectly dry, moist air is lighter than air containing {134} little or no moisture. Hence the barometer falls as the quantity of atmospheric moisture approaches saturation. Other causes, however, also affect barometric pressure. With the same degree of humidity, cold air is denser and heavier than warm air, and by its contraction lowering the "column" of atmosphere--the temperature of which is reduced--a flow toward the upper part of the column increases the actual mass of air pressing upon a particular place. Elevation of a locality above the general level of the earth reduces atmospheric pressure, sensibly as well as measurably. So "the difficult air of the iced mountain-top" has become proverbial. These variations are familiar, though all their effects upon human health have been by no means, as yet, fully studied. Most difficult to determine and analyze are the influences of changes of pressure, chiefly hygrometric, upon the course of diseases and upon the result of severe surgical operations. Among the few important series of observations bearing on this topic have been those of Dr. S. Weir Mitchell on neuralgia,[4] and Dr. Addinell Hewson on the prognosis of major operations,[5] in connection with the state of the weather. The former ascertained a marked relation between the approach of a wave of low barometric pressure and attacks of irregularly periodic neuralgia; the latter proved, by the statistics of the Pennsylvania Hospital for a number of years, that the most favorable time for amputations or other capital operations is when the barometer is high, or at least on the ascent. [Footnote 4: _American Journal of Medical Sciences_, April, 1877, p. 305.] [Footnote 5: _Pennsylvania Hospital Reports_, 1868.]