ANEROID BAROMETER. At the Swan sea meeting of the British Association in 1848, Professor Lloyd introduced a notice of the Aneroid Barometer, which had been patented by M. Foutainmoreau in 1844. Instead of acting by the pressure of the atmosphere on liquids, as most forms of barometer do, this instrument depends on the pressure of the air on thin lamina; or diaphragms of some elastic solid substance. The body of the in strument consists of a hollow brass box, with a dial face on the upper surface. Below the dial face is the diaphragm, stretched all across the box. This may consist of a thin sheet of metal, or of glass, or of caoutchouc, or of some other elastic material ; but the patentee men tions especially a thin piece of copper sheet, corrugated circularly, so as to yield sensibly to any pressure. The box beneath the dia pliragm is so made that the air can be ex tracted from it ; and the diaphragm is then only protected from the crushing pressure of the air by a number of delicate spiral springs, which are fixed to the interior of the bottom of the box, and support the diaphragm. When the weight of the atmosphere increases (equi valent to the rising of the ordinary barometer) the diaphragm suffers an increased pressure, to which the springs allow it to yield to a cer tain extent; but a lessened pressure allows it to rise again to its normal state. A delicate piece of mechanism, connected with the centre of the diaphragm, becomes moved when the diaphragm either rises or sinks; and this move ment is transferred to index hands, which mark on a graduated dial the amount of rise or fall. The graduations are so made as to cor
respond with the inches and fractions of an inch in the common barometer.
From a communication made to the Alden (cum (No. 1117), it appears that an Aneroid barometer was carefully compared for several days with the standard barometer at the Liver pool Observatory. The greatest difference was about 'four-tenths of an inch ; the aneroid showing 31.22 inches on an occasion when the standard barometer showed 30-80. The least difference was one-fiftieth of an inch, the quantities being 29.22 (aneroid) and 29.24 (standard). At a high temperatures or a high pressure, the aneroid was generally in excess of the standard barometer ; at a low tempera ture or a low pressure they were nearly on an equality.
It is still a question of doubt among scientific men, to what extent the aneroid barometer may be relied on in various seasons and climates. With respect to the originality of the invention, it is known that something nearly analogous was suggested long ago. The more ordinary instruments for this purpose are described under BAROMETER.