panied by more or less abrupt changes in the temperature and the humidity. Mountain ranges also produce changes in the meteorologic conditions of the surrounding country. These changes vary with the height of the summits, the extent and trend of the range, and the nature and abundance of the vegetation. Mountain-chains may serve as barriers to cold and to hot winds, and may, consequently, cause an elevation or depression of the annual temperature of the contiguous regions.

They exert, moreover, an influence upon the winds themselves, causing a condensation of the watery vapor when they, the winds, are heavily charged with humidity. They may thus render the climate of one place dry and of another more moist.

The local condition, being so very variable, should be specially studied for each place.

In regard to places situated directly on a mountain, the most important point for investigation is whether they are upon the sunny or the shady side. In the northern hemisphere the southwest side is generally the hottest, and the northeast the coldest. The west side is a little warmer than the east.

The prevailing direction of the wind should also be inquired into, for this is of even greater importance than the exposure of the place. Indeed, it is upon this that the conditions of humidity especially depend. In Europe the southwest declivities are generally more moist than the southeast, although they receive more heat.

The surface of the soil has an important influence upon the temperature and humidity of the lower strata of the atmosphere. The experiments of Frankland have revealed certain relations between the nature of the soil and the solar heat.

Snow offers the strongest reflecting surface; after it, in a descending order, may be named light-colored soil, dried grass, gray rocks, and green grass.

The absorbing power of the soil is in inverse ratio to its radiating power. The lighter the color of the surface the greater is the solar heat and the cooler is the atmosphere. A dark soil is associated with a warm atmosphere, while the radiant heat is proportionately less great.

The power of absorbing moisture is still more variable according to the nature of the soil than the heat-absorbing power. Elliott states that turf absorbs more than twice its weight of water, dry clay its own weight, dry garden soil half its weight, sand a little more

than a third. This explains why sand dries so quickly and turf so slowly.

Buchan has proved that drainage influences to a marked degree the temperature of the soil. He found that it elevates the temperature of arable land approximately one degree. Cold penetrates more rapidly through undrained than through drained land. Irrigated soil loses its heat less readily than soil which is not irrigated; such a soil has also more constant temperature. These facts may elucidate the diminution of phthisis, which Bowditch, Buchan, and others have observed to follow the establishment of drainage.

The presence or absence of vegetation, and the nature of this vegetation, modify in a marked degree the factors of climate.

The vegetation which covers the earth hinders more or less effectually the solar rays from reaching the surface of the soil, and thus moderates the temperature in a variable degree. On the other hand, plants themselves, by reason of the constant evaporation from them, tend to modify the temperature.

The effects produced by the vegetation of a region varies with the character which this vegetation generally assumes, namely, whether it be forest, meadow, or turf.

We owe to Fautral and to Erlenmeyer the greater part of the knowledge we possess concerning the influence of forests.

The soil of forests has a lower temperature than that of prairies to a depth of one yard, the difference being more marked in summer than in winter. The temperature of the air of forests is also lower than that of the prairie. We all have found during the noonday heat delightful coolness in the forest shade. The temperature-variations are less abrupt in forests than on plains, and the climate is more constant.

The days are cooler and the nights warmer, but the difference between the maximum and minimum temperature is less marked. The humidity is always greater than it is on the plain, exceeding that of the latter by nearly 10° in July, and 3° to 7° in January. Rain is therefore more abundant in forest regions than in those denuded of their trees. Lastly, forests may arrest winds, and thus exert an appreciable influence at distances more or less great. Regarding the climate of cultivated plains, it is well to know that it is never so warm as that of regions from which vegetation is absent. H. Weber has noted a difference of 11° to 17° between the soil of a barren district and that of an adjoining plain.

Peat or turf lands and swamps are characterized by a high degree of humidity of the soil and air, and by a relatively low annual temperature. Fogs are not common. The air in these marshy districts frequently harbors the poison of malaria, which may be wafted to distant regions according to the direction of the wind and the configuration of the surface.

From another point of view, the vegetation of a place furnishes a valuable indication of its climate, and we may, indeed, say, with Ch. Martins, that plants are living thermometers; or, with Fonssagrives, that they are truly climatometers.

The investigation of the flora of a country is, therefore, an important study.

From the therapeutic standpoint, the climatical and astronomical seasons do not exactly coincide. Summer comprises the months of June, July, and August, and stations visited during these months are known as summer stations. December, January, and February are the winter months, and stations at which they are passed are known as winter stations. The other seasons correspond to the intermediate months.

Of the various elements or factors of climate the first to be considered is the constitution of the atmosphere, which is obviously of the greatest importance, since the air we inspire is the very principle of life, and upon its qualities depend, in a large measure, health and disease. Its importance attracted the attention of the great physicians of antiquity, notably of Hippocrates.

Concerning the subject of chemical constitution we find that the subject still presents many obscurities. The pressure being equal, the proportion of the gaseous principles is almost constant. The slight variations in the quantity of oxygen have no visible influence upon health.

The quantity of carbonic dioxide contained in the air is, as a rule, very small, but may at times be augmented without becoming harmful. This increase, however, deserves attention, since it is correlated with the presence in the air of heterogeneous principles that may be

noxious.

Another constituent of the air is ozone, which chemists regard as a combination of oxygen with itself (an allotropic form of oxygen). The presence of ozone bears some relation to the electric phenomena of which the atmosphere is at times the scene, for frictional (static) electricity, to which that of the air belongs, ozonizes the atmosphere.

Although the proportion of ozone is normally very small, 1 in 10,000 as a maximum, there is nevertheless a certain importance attributed to it. Not that this form of oxygen is capable, in such small amounts, of influencing the organism to any appreciable extent, but the conditions in which an increase is noted seem to be favorable to good hygiene. Thus it is not present where putrefaction prevails, nor in hospitals. In the interior of cities the quantity of ozone is less than in the country; more is found in the air along the seacoast than further inward, and likewise more in the mountain atmosphere than in that of the plains. The only sensible effect of small quantities of ozone consists in an increased tendency to sleep. The humidity of the atmosphere is also of great importance to the study of effects of climate.

Beside the gases proper the atmosphere always contains a certain quantity of watery vapor, and it is this constituent which, from the climatic standpoint, is the most important.

The proportion of watery vapor contained in a given volume of air depends upon the temperature. Two forms of humidity, an absolute and a relative, must be distinguished. By the first is implied the quantity of vapor contained in a certain volume of air; the second indicates the relation between the quantity of vapor and that which the air would contain if it were saturated. When we speak of dry air or moist air we refer to the fact that the relative humidity is respectively low or high. The absolute humidity varies with the seasons, and obeys, up to a certain point, the fluctuations of temperature. It is more elevated in summer than in winter. The variations in relative humidity follow almost a directly opposite course, and for our purpose the greater interest centres in them.

According to Humboldt, the lowest degree of relative humidity is 23 per cent. The air is very dry when the relative humidity is under 35 per cent., moderately dry when it rises to from 55 or 75 per cent., moderately humid when it is from 75 to 90 per cent., very humid at 90 to 100 per cent.

The humidity varies not only according to the seasons, but also according to the nature of the winds, particularly when these come from the sea, and according to the different periods of the day. It is generally highest at sunrise, the minimum being reached during the early afternoon hours.

Closely related to atmospheric humidity are several interesting meteorological phenomena, such as fog, clouds, and rain. Fog and

clouds moderate the variations of temperature of a region, and consequently play a part in the determination of the climate.

The influence of rain must be considered from several points of view. The quantity which falls during a year is extremely variable. It is estimated by means of pluviometers (rain-gauges). From nothing on the Sahara it rises to nearly twelve yards on the southeastern declivity of the Himalaya Mountains.

In medical climatology account should be taken of the number of rainy days in a year and in each month. There is no relation between this number and the quantity of rain. It is also important to note the hours during which rain falls, for upon this fact depends

the length of time the patient can remain in the open air.

In the mountains the rain is replaced by snow, which, happily, quite often falls only toward evening or at night.

Despite the deprivation of water, represented by the snow or the rain, the atmospheric air preserves its humidity.

The degree of evaporating power of the atmosphere depends upon a number of conditions, such as the temperature, the relative humidity, the density, and the velocity of the wind. This power possesses considerable physiological importance, since it regulates the cutaneous evaporation, and, in part, the loss of heat from the surface of the body. The atmospheric humidity is thus as essential to life as oxygen itself. It determines the distribution of heat on the surface of the earth, and also prevents the warm air from rising too rapidly into the higher strata of the atmosphere and being thus lost to us. The vapor of water is practically opaque; it prevents the radiation of heat. This explains why the condensation of vapor consequent upon the cooling of the soil constitutes a means of protection. Humid climates for this reason possess the advantage that the thermic variations between the day and night are but slight, and that the temperature is more constant. The excess of humidity is, however, in some respects inconvenient, for it intercepts the light and heat and produces a depressing effect upon the system.

The action of humidity on the human body is very noticeable, but difficult to define on account of its complexity. Thus the degree of absolute humidity is an important factor as regards the respiratory functions. The dryer the air the more moisture it absorbs in its passage through the air-passages. Hence the diminution of the expectoration in a dry climate.

The relative humidity produces its effects especially upon the func