approaches more nearly the point of saturation, and from the increased elasticity, the pressure rises to the evening maximum. As the deposition of dew proceeds, and the fall of temperature and consequent downward movement of the air are arrested, the elasticity is again diminished, and pressure falls to the morning minimum. Since the view propounded some years ago, that if the elastic force of vapour be sub tracted from the whole pressure, what remains will show only one daily maximum and minimum, has not been con firmed by observation, it follows that the above explanation is quite insufficient to account for the phenomena ; indeed, the view can be regarded in no other light than simply as a tentative hypothesis. Singularly enough, Lamont and Broun, a few years ago, were led, independently of each other, to form an opinion that the daily barometric oscillations were due to the magneto-electric influence of the sun. It admits of no doubt, looking at the facts of the case so far as they have been disclosed, that the daily barometric oscillations originate with the sun, and that more than the sun - s influence as exerted on the diurnal march of the tempera ture and humidity of the atmosphere is concerned in bringing them about. But from the facts adduced, it is equally certain that, be the originating cause what it may, its effects are enormously modified by the distribution of land and water over the globe, by the wind, and by the absolute and relative humidity of the atmosphere. The smallness of the amount of the summer oscillation from the forenoon maximum to the afternoon minimum over the North Atlantic as far south as lat. 30, and its diminished amount, as far south at least as the equator, will no doubt play an important part in the unravelling of this difficulty. One of the most important steps that could be taken would be an extensive series of observations from such countries as India, which offers such splendid contrasts of climate at all seasons, has a surface covered at one place with the richest vegetation, and at others with vast stretches of sandy deserts, and presents extensive plateaus and sharp ascending peaks all which conditions are indis pensable in collecting the data required for the solution of this vital problem of atmospheric physics. The ancients thought that air was one of the four elements from which all things originated, and this doctrine continued to prevail till 1774, when Priestley discovered oxygen gas, and showed it to be a constituent part of air. Nitrogen, the other constituent of air, first called azote, was discovered soon after, and the marked differences between these two gases could not fail to strike the most careless observer. It is remarkable that Scheele independently discovered both oxygen and nitrogen, and was the first to enunciate the opinion that air consists essentially of a mixture of these two gases. From experiments made by him to ascer tain their relative volumes he concluded that the propor tions are 27 volumes of oxygen and 73 volumes of nitrogen. It was left to Cavendish to show from 500 analyses that the relative proportions were practically constant, and that the proportion is 20 "8 3 3 per cent, of oxygen. The results obtained by Cavendish, though not attended to for many years after they were published, have been shown by recent and more refined analyses to be wonderfully exact. The most recent analyses of specimens of air collected under circumstances which ensure that it is of average purity, give as a mean result the following : Volume. Oxygen ............................ 20 96 per cent. Nitrogen ............................ 79 00 Carbonic acid ..................... 04 100-00 The circumstances under which these proportions vary, and the other gases and substances which are found in the air, will be afterwards adverted to. Besides these three constituents of air, there is a fourth, viz., the vapour of water, from which no air, even at the lowest temperatures yet observed, is wholly free, so that absolutely dry air does not exist in the free atmosphere. The dry air of the atmosphere oxygen (inclusive of ozone), nitrogen, and carbonic acid is always a gas, and its quantity is constant from year to year; but the vapour of water does not always remain in the gaseous state, and the quantity present in the atmosphere is, by the processes of evaporation and condensation, varying every instant. Water evaporates at all temperatures, even the lowest, and rises into the air in the form of an invisible elastic gas called aqueous vapour. The elasticity of vapour varies with the temperature. At Fahr. it is capable of sustaining a pres sure equal to 044 inch of the mercurial barometer, as calcu lated from Kegnault s experiments; at 32 (freezing), 181 inch; at 60, 518 inch; at 80, T023 inch; and at 100, 1 918 inch, being nearly ^ the average pressure of the atmosphere. In investigating the hygrometry of the atmosphere, the chief points to be ascertained are (1), the temperature of the air; (2), the dew-point; (3), the elastic force of vapour, or the amount of barometric pressure due to the vapour present ; (4), the quantity of vapour in, say, a cubic foot of air; (5), the additional vapour required to saturate a cubic foot of air; (G), the relative humidity; and (7), the weight of a cubic foot of air at the pressure at the time of obser vation. The vapour of the atmosphere is observed by means of the hygrometer (see HYGROMETER), of which it is only necessary here to refer to ReynaulCs as the most exact, and August s as the most convenient, and, consequently, the one in most general use. August s hygrometer consists of a dry and a wet bulb, with which are observed the tempera ture of the air and the temperature of evaporation. Of these two observed data, the formula of reduction, as deduced from Apjohn s investigations, is as follows : Let F be the elastic force of saturated vapour at the dew-point, / the elastic force at .the temperature of evaporation, d the difference between the dry and wet bulb, and b the barometric pressure, then d b TV / v ~ 7 88 30 when the reading of the wet bulb is above 32 ; and i7 d b ==/ ~96 X 30 when below it. From Kegnault s values of the elastic force of vapour, / is found, and d and b being observed, F is calculated. From F the dew-point is found. In calculat ing relative humidity, saturation is usually assumed to be 100, perfectly dry air 0. The humidity is found by divid ing the elastic force at the dew-point by the elastic force at the temperature of the air, and multiplying the quotient by 100. The elastic force may be regarded as representing approximately the absolute quantity of vapour suspended in the air. It may be termed the absolute humidity of the atmosphere. Since the chief disturbing influences at work in the atmosphere are the forces called into play by its aqueous vapour, a knowledge of the geographical distri bution of this constituent through the months of the year is of the utmost possible importance. Hence every effort ought to be made to place the observation of the hygrometry of the air, and the reduction of the observed data, on a sounder basis than has yet been done. As regards geogra phical distribution, the elastic force is greatest within the
tropics, and diminishes towards the poles ; it is greater over
