BY HEAT OF WATEE AXD MEllCUEY, 233 



upward pulling of the spring. Similar springs (k, k, /r, k) were fixed to the glass rod in 

 the trough, to keep it pressed against the end of the fixed glass tube. 



The whole of the apparatus stood on a brick floor, and so solid was the founda- 

 tion, that accurate measurements could be made even when persons were moving about 

 on it. 



The measuring-apparatus* (D, fig. 1) was a micrometer-screw, a piece of brass being 

 fixed to the end of it, through which passed a moveable circular piece of the same alloy 

 faced with agate. On turning the screw, this piece of brass, as soon as it touched the 

 zinc cap, pressed against the short arm of the lever (I), and thus indicated the point of 

 contact. To obtain accurate readings, the screw was turned until the bent end of the 

 long arm of the lever covered the line on the table beneath it ; and to avoid parallax, the 

 point was observed through the tube E, which has a very fine opening at the top and a 

 magnifjing glass at the bottom. 



With this arrangement readings can be made with little practice (of course avoiding 

 the back-lash of the screw), with great accuracy, in fact, easily to 0-001 millimetre. 



The length-value of a turn of the micrometer-screw was found by measuring wires of 

 known diameter. These were measured some years since in Heidelberg, in Kirchhoff's 

 laboratory, with a calibrated micrometer-screw under a microscope. I thought, how- 

 ever, the screw might have altered by constant use, as it had served to measure all the 

 wires used for the electrical conducting-power experiments made in conjunction with 

 other gentlemen. On applying to Mr. B. Stewart, he kindly placed at my disposal, 

 at Kew, their normal measuring- screw, and on redetermining the length- value of 

 a turn of the screw, it was found equal to 0-185 millimetre, agreeing almost abso- 

 lutely with the former value, namely 0-184 millimetre. The mean of these two values 

 was used, namely 0-1845 millimetre. The milled head was divided into 200 parts, so 

 that each division represents about 0-001 millimetre. A small correction had to be 

 made for the movement of the lever, as it moves bodily Avitli the screw. The relative 

 lengths of the arms of the lever were about as 250 : 1, so that when the screw moved 

 backwards to allow the rod to expand, the error caused by this in the reading will be 

 about 0-005 millimetre, for the expansion of the rod is equal to 1-3 millimetre ; in other 

 words, to bring the point of the lever to cover the line at the high temperatures, the 

 screw would be turned too much, or the expansion would be found too small. By direct 

 determination this correction amounts, for six turns of the screw, to 0-005 millimetre. 



On carefully calibrating the screw, a part of it (consisting of eight turns) was found 

 where the length-value of the threads was almost absolutely the same. This portion of 

 the screw was therefore used for the experiments. The thermometers employed were 

 correct, having been compared with a Kew Standard thermometer. Their zero-points 

 were redetermined from time to time. The correction for the column of mercury not 

 Immersed in the water, made by using Kopp's formula, 



correction = N(T-0 X 0-0001545, 

 * This apparatus was made bj- Mr. Becker for the accurate measurement of the diameters of wires. 

 MDCCCLXVI. 2 K 



