We are indebted to the weight of the atmosphere for the power we possess of raising water by the common pump; for the piston of the pump withdrawing the air from the interior of the pipe, which terminates in water, the pressure of the atmosphere forces the water up the pipe to supply the place of the air withdrawn. It was soon found, however, that when the column of water in the pipe was more than thirty feet high, the pump became useless, for the water refused to rise higher. Why? It was found that a column of water about thirty feet high exerted a pressure equal to the weight of the atmosphere, thus establishing an equilibrium between the water in the pipe and the atmospheric pressure. This is the principle on which the barometer, or _measurer of weight_, as its name imports, is constructed. The metal Mercury is about thirteen and a half times heavier than water; consequently, if a column of water thirty _feet_ high balances the pressure of the atmosphere, a column of mercury thirty _inches_ high ought to do also--and this is in fact the case. If you take a glass tube nearly three feet long, and closed at one end, and fill it with mercury; then, placing your finger on the open end, invert the tube into a basin or saucer containing some of the same metal; upon removing your finger (which must be done carefully, while the mouth of the tube is completely covered by the mercury), it will be seen that the fluid will fall a few inches, leaving the upper part of the tube empty. Such a tube with a graduated scale attached is in truth a barometer, and as the weight of the atmosphere increases or decreases, so the mercury rises or falls in the tube. This instrument is of the greatest value to the seaman, for a sudden fall of the barometer will often give notice of an impending storm when all is fine and calm, and thus enable the mariner to make the preparations necessary to meet the danger. It was discovered by an Italian philosopher named Torricelli, and from him the vacuum formed in the upper end of the tube above the surface of the mercury has been called the Torricellian vacuum. It is by far the most perfect vacuum that can be obtained, containing necessarily nothing but a minute quantity of the vapor of mercury. EXPERIMENT. Pass a little ether through the mercury in the tube, and as soon as it reaches the empty space it will boil violently, depressing the mercury, until the pressure of its own vapor is sufficient to prevent its ebullition. If you now cool the upper part of the tube, so as to condense the vapor, the pressure being thus removed, the ether will again begin to boil, and so alternately, as often as you please. In order to show this fact with effect, the bore of the tube should not be less than half an inch in diameter. EXPERIMENT.