PIEZWIETER (in(Cca and perpeco). Under ELASTICITY are noticed the various attempts to measure the compressibility of water and other fluids, and reference is made to Professor Oersted's instrument by which the problem wassatisfactorily solved. It consists of a strong glass cylindrical vessel a ne n with a smaller cylinder sass. screwed into the top, containing an air-tight piston x worked by a screw; Al is a glass bottle, in to the neck of which is ground a capillary tube a a, open at both ends, of fine bore, and each inch of its length must express a precise fraction of the contents of the bottle. This is done by weighing hew much mercury the bottle will hold, and how much an inch of tho boro of the tube. In some of Oersted's tubes each inch in length was found to hold of the contents of the bottle. Let us now suppose the whole of the apparatus to be filled with water : any pres sure upon it which causes its surface in the tube a a to descend one inch, will have compressed it by of its bulk. The scale attached to the tube was graduated, each inch into 40 parts, so that a depression of the water in the tube through one of these divisions would indicate a compression of two-millionths. The apparatus being arranged as in the figure, only with the piston K at the bottom of the short cylinder in contact with the water which fills the large cylinder and the bottle ra, the piston is screwed back towards EY, and as it recedes water is drawn into the vessel by the, siphon 13 r from the vessel r. A cock is then closed in the siphon, and on screwing the piston downwards the pressure is propagated through the water in the cylinder, and also through the tube a a to the water in the bottle, such pressure being of course precisely the same within and without the bottle. A separation is made between the water in the tuba a a and that in the large cylinder, by means of a bead of air contained in the glass cap r, which yielding to the pressure descends the tube a a, and serves as an index finger to the scale. The amount of pressure
applied by driving down the piston is measured by means of the tube I:, which is closed at the top, open at the bottom, and graduated into equal parts. It has a ring of lead at the bottom to keep it immersed, and being full of air it readily indicates by its compression the amount of pressure applied to the water by the piston. By the law of Marriotte [ELASTICITY], the diminution in the bulk of aeriform bodies is proportional to the pressure, so that by observing the degree to which the air is in the tube by the rise of water in it, the pressure exerted by the piston and that sustained by the water in the bottle is always known. In this way it was ascertained that water is compressed millionths of its bulk for every atmosphere of 15 lbs. on the square inch. Of course by filling the apparatus with some other liquid its compressibility could be ascertained, and taking mercury as unity in millionths of an inch, alcohol was found to be 20, sulphide of carbon 30, water sulphuric ether 60.
The results obtained by Messrs. Colladon and Sturm differ considerably from those of Oersted. Those physicists state that a correction should be made for the com pression of the substance of the tube and bottle, in consequence of the pressure sustained by them within and without. 31. Poisson has given a simple rule for this correction, the necessity for which is denied by Oersted, who states that the recession of water in the capillary tube is about 11 millionths greater when bottles of lead an,' tin are used instead of bottles of glass. Piezometers with bulbs of different ma terials, filled with some liquid of known compressibility, have been used for ascertaining the cubic compressibility of the material of the bulbs. Thus it has been ascertained by Professor Forbes, that the cubic compressibility of caoutchouc is about the same as that of water, for when a piezometer with a caoutchouc bulb filled with water is compressed, the index remains nearly stationary in the stem.