The Edinburgh Encyclopaedia Volume 18 PDF Download

Excerpt from The Edinburgh Encyclopaedia, Vol. 18 of 18 Boyle therefore subsequently proposed to include both the air and the coloured fluid in one bulb as shown at Fig. 2, where the tube AB which is open at top, reaches below the surface of the fluid nearly to the bottom of the receptacle C, into the neck of which the tube is hermetically sealed. The same philosopher, however, demonstrated the grand defect of the air thermometer, that by a change of pressure in the atmosphere, as shown by the barometer, the elasticity of the inclosed air is altered independently of temperature, so as to render the indications of the same instrument not comparable at different times. The air thermometer was subsequently modified by the ingenious Hooke, in order to act as a barometer, which it obviously does, if the effects of temperature be corrected and those of pressure alone shown, just as on the other hand, if the result of variable pressure were neutralized, that of temperature would be truly expressed, Hooke attached a mercurial thermometer to the original instrument, the temperature of which thus indicated gave the data for separating the influence of dilatation caused by heat, which was performed by means of a sliding scale. By this elegant modification Hooke converted the air thermometer into a marine barometer, which, however, was soon abandoned from the absorption that was found to take place of the excluded air by the coloured fluid. This defect has more lately been in a great measure remedied by the substitution of hydrogen gas instead of the common included air, by Mr. Adie, who has revived this instrument under a very elegant and portable form, and under the name of the sympiesometer. See Meteorology. The only other air thermometer we intend to notice is that of Amontons, who made the indicial fluid a column of mercury twenty-eight French inches long, so that the included air was subjected to the pressure of two atmospheres; by this method he was enabled to measure high temperatures, such as that of boiling water, without a scale of such great length as the dilatation of air under the common pressure would have required. It was, however, subject to the same great defect as the Sanctorian thermometer, and was besides very unwieldy and liable to accident. In the seventeeth century a modification of the air thermometer was proposed by Van Helmont and by Sturmius, which has recently been revived under the names of Thermoscope and Differential thermometer; as this, however, does not belong to the simple form of the instrument, we shall give some account of it in another part of this article. The defects of the air thermometer having been duly appreciated by the Florentine. Academia del Cimento, that enterprising body published, in the first volume of their transactions, a description of a new thermometer, in which spirit of wine was used as the expanding substance, which, as it might be hermetically sealed up in a glass tube or bulb, was free from any defect arising from pressure, as well as the possibility of any loss of fluid by evaporation. This instrument was constructed much in the same way as at present, the spirit being dilated till it filled the whole tube, when it was quickly sealed, and on cooling, the fluid retired, leaving nearly a vacuum above it. The great defect of the Florentine weather glass, as it was commonly called, was the want of any fixed scale of graduation, on which account no instrument except those graduated by the original one of the academy, could be comparable with any other, the only direction being that the cold of ice and snow should make it stand at 20 deg. and the greatest summer heats at Florence, at 80 degrees. The spirit thermometer was faulty in several other respects, yet it cannot but be thought fortunate that this fluid, which is esteemed the second best for filling thermometers, should have been so early thought of. The Florentine academicians sometimes bent the ."