This instrument may be employed as a marine ba rometer; but the friction of the mercury is so great, owing to the bore of the tube being necessarily very small, that it is seldom used.
The statical barometer of Otto de Guericke, Boyle, and others, consisted of a large hollow sphere fixed at one end of the arm of a delicate balance, counter poised by a weight of brass at the opposite. These two bodies being of the same weight, but of different volumes, if the fluid or medium in which they are suspended becomes more or less dense, an apparent change of weight will take place, and the equili brium will be subverted. If the air becomes heavier, the hollow ball will appear to become lighter, as it will lose more of its weight than the counterpoise, which is more denie ; on the other hand, if the air becomes lighter, a contrary effect will happen. This barometer, is obN!'ausly founded on the hydrostatical principle, that a body suspended in a fluid loses as much of its weight as is equal to the weight of the fluid displaced. il. Trans. 1666. No. 14.
Fahrenheit proposed a barometer, founded, in principle, on the well-known fact, that the boiling point of liquids varies with the pressure on their sur face. If a thermometer be taken with a large bulb, and a small bore, and the boiling points of water be marked upon it, corresponding to the various heights of the mercury in the Torricellian tube at the time of observation, the divisions on the thermometer will indicate the pressure of the air, when the instrument is afterwards plunged into boiling water. The range of the scale, however, would be very limited ; as a change of atmospherical pressure, causing a descent of one inch of the mercury in the barometer, depres ses the boiling point of water only 12 degrees ; the instrument would also be troublesome in application.
In the Philosophical Transactions, a barometer of the following construction is described by Mr Cas well : ABCD is a vessel filled with water, in which is immersed the barometer msoyzerx, consisting of a body msoerx, and two tubes cx and oyze. The
body and the lower tube are hollow cylinders, and communicate with each other.. The lower extremity of the tube yz has a weight affixed to it, to make the instrument sink, so that the top of the body may just swim even with the surface of the water, by the addition of necessary weights on the top. When the instrument is forced with its mouth downwards, the water ascends up into the tube to the height ut; and the small concave cylinder cx at the top gives buoy ancy to the whole, and prevents the instrument from sinking below the proper depth ; sod is a wire ; and and cd arc two threads stretched obliquely to the surface of the water, in order to increase the range of the scale. An alteration of the gravity of the air causes the instrument to subside more or less ; and a small bubble is formed "where the surface of the wa ter cuts the threads, which ascends and descends along them, as the mercury ascends and descends in the common barometer. This instrument is much commended by its inventor for its extreme delicacy ; but the difficulty of applying an accurate scale to it, renders.it of little value.
On the whole, it may be remarked, that the prin ciple of the Torricellian experiment affords the best method of constructing the barometer with accuracy ; particularly as, by means of a vernier scale, the height of the mercury may be readily determined to the thousandth part of an inch ; a degree of correctness sufficient for every scientific and practical purpose, and which cannot be obtained with certainty by con structions of a more complicated nature.
The references to works on the subject of barn` incurs will be given with more propriety under the articles HEIGHTS and METEOROLOGY. (A)