=1. Principle of the Barometer.=--The first instrument which gave the exact measure of the pressure of the atmosphere was invented by Torricelli, in 1643. It is constructed as follows:--A glass tube, CD (fig. 1), about 34 inches long, and from two to four-tenths of an inch in diameter of bore, having one end closed, is filled with mercury. In a cup, B, a quantity of mercury is also poured. Then, placing a finger securely over the open end, C, invert the tube vertically over the cup, and remove the finger when the end of the tube dips into the mercury. The mercury in the tube then partly falls out, but a column, AB, about 30 inches in height, remains supported. This column is a weight of mercury, the pressure of which upon the surface of that in the cup is precisely equivalent to the corresponding pressure of the atmosphere which would be exerted in its place if the tube were removed. As the atmospheric pressure varies, the length of this mercurial column also changes. It is by no means constant in its height; in fact, it is very seldom stationary, but is constantly rising or falling through a certain extent of the tube, at the level of the sea, near which the above experiment is supposed to be performed. It is, therefore, an instrument by which the fluctuations taking place in the pressure of the atmosphere, arising from changes in its weight and elasticity, can be shown and measured. It has obtained the name _Barometer_, or measurer of heaviness,--a word certainly not happily expressive of the utility of the invention. If the bore of the barometer tube be uniform throughout its length, and have its sectional area equal to a square inch, it is evident that the length of the column, which is supported by the pressure of the air, expresses the number of cubic inches of mercury which compose it. The weight of this mercury, therefore, represents the statical pressure of the atmosphere upon a square inch of surface. In England the annual mean height of the barometric column, reduced to the sea-level and to the temperature of 32 deg. Fahrenheit, is about 29.95 inches. A cubic inch of mercury at this temperature has been ascertained to weigh 0.48967 lbs. avoirdupois. Hence, 29.95 x 0.48967= 14.67 lbs., is the mean value of the pressure of the atmosphere on each square inch of surface, near the sea-level, about the latitude of 50 degrees. Nearer the equator this mean pressure is somewhat greater; nearer the poles, somewhat less. For common practical calculations it is assumed to be 15 lbs. on the square inch. When it became apparent that the movements of the barometric column furnished indications of the probable coming changes in the weather, an attempt was made to deduce from recorded observations the barometric height corresponding to the most notable characteristics of weather. It was found that for fine dry weather the mercury in the barometer at the sea-level generally stood above 30 inches;