DIFFERENTIAL THERMOMETER. An ingenious instrument of great use in experimental philosophy, for measuring very small differences of temperature ; invented and re-applied by Sir John Leslie, though the idea of an instrument of the same kind seems to have long fore been suggested by Sturmins. The differential thermometer is described by Leslie, in his Experimental Inquiry into the Nat ure and Propagation of Heat, nearly as follows : Two glass tubes of unequal lengths, each terminating in a hollow ball, and having their bores somewhat widened at the other ends, a small portion of phuric acid tinged with carmine being troduced into the ball of the longer tube, are joined together by the flame of a pipe, and afterwards bent into nearly the shape of the letter U; the one flexure being made just below the joining, where the small cavity facilitates the adjustment of the instrument, which, by a little terity, is performed by forcing with the beat of the hand a few minute globules of air from the one ball into the other. The balls are blown as equal as the eye can judge, and from four-tenths to tenths of an inch in diameter. To one of the legs of the thermometer a scale is attached ; and the liquid in the tube is so disposed that it stands in the ated leg opposite the zero of the scale, when both balls are exposed to the Caine temperature. From
this construction of the instrument, it is easy to see that it is affected by the difference only of heat in the two balls. As long as both balls are of the same temperature, whatever this may be, the air contained in the one will have the same elasticity as that contained in the other; and consequently the inter eluded colored liquid, being thus pressed equally in opposite directions, remains sta tionary. But if, for instance, the ball which holds a portion of the liquor be .warmer than the other, the superior elas ticity of the confined air will drive it for wards, and make it rise in the opposite branch above the zero, to en elevation proportional to the excess of elasticity or of heat. Sulphuric acid is chosen as the liquor best adapted to the purpose ; be cause it is not vaporizable, and conse quently does not by its vapor affect the pressure of the air above it. The car mine is used to render it more easily visible.