228 PHILOSOPHICAL TKANSACTIONS. [aNNO 1777. 



tedious, and now wliolly useless, since it was from those of the 3d class alone, 

 that the rate as well as maximuui of expansion was ascertained. Having, in 

 different ways, made a great variety of ingenious and nice experiments, some 

 with tubes of more than -i% of an inch in diameter, of the ordinary length, 

 with a vacuum over the quicksilver of 2-1- or 3 inches, part of which reached 

 above the top of the vessel. The mean of 3 experiments gave .5258, for the 

 total dilatation of 30 inches of quicksilver, on 1 80° between freezing and boil- 

 ing ; that answering to the first 20°, between 32° and 52°, was .o688 ; that for 

 the 20° in the middle of the scale, between 112° and 132°, was .058 ; and the 

 rate for the last 20°, between 102° and 212", was only .041. From this first set 

 of the 3d class of experiments, it appeared evident, that the expansion of 30 

 inches of quicksilver in the barometer, suffering a heat equal to 180° of Fah- 

 renheit, instead of exceeding Mr. De Luc's, as appeared to be the case from the 

 results of the open tube, really fell short of it : and instead of being arithmeti- 

 cal or uniformly the same, for equal changes of temperature, was actually pro- 

 gressive ; the expansion answering to the lower part of the scale, being greater 

 than that corresponding to the middle ; which again exceeded that for high tem- 

 peratures. In these experiments, when the water had acquired a heat 20 or 30 

 degrees greater than that of the open air, a certain dustiness was perceived in the 

 vacuum of the tube. About 100° of Fahrenheit this appearance had so far in- 

 creased, as to show clearly, that it could proceed from no other cause than a 

 vapour arising from the surface of the heated quicksilver, quite invisible, till, by 

 its condensation in the cold part of the tube, it was formed into balls, every 

 where adhering to its sides and summit. These globules were very small near 

 the surface of the water, but augmenting gradually as they approached the top 

 of the tube, where they were greatest : their bulk increased with the heat ; and 

 when the w?ter was at or near boiling, they would sometimes unite, and descend 

 by their own gravity, along the sides of the tube, into the general mass. 

 Hence the progressive diminution of the rate of expansion of the column of 

 quicksilver in the barometer, is easily accounted for by the resistance of the 

 elastic vapour,* acting against the top of the tube, which was here colder than 

 the rest. 



But in the application of the barometer to the measurement of heights, the 

 whole instrument is of the same temperature; therefore, in the 2d set of this 

 3d class of experiments, the tin vessel was heightened, that tubes of the ordinary 

 length, placed in it, might be wholly immersed in boiling water. The mean of 

 4 experiments, which agreed very nearly among themselves, gave .5117 for the 



* Having mentioned the circumstances to Mr. Ramsden, it first occurred to him, that tlie resijt- 

 ance of the elastic vapour was the cause of the dimLuution in the rate of expansion. — Orig. 



