360 Mr A. J. Adie on the Expansion of Stone 



ment and back again to the chimney passed through holes in 

 the screen-boards. A detached thermometer was hung over the 

 instrument, and it was always kept at the same height by admit- 

 ting more or less air by the window. In order to have the same 

 direction of light during each experiment, I employed two small 

 lamps, placed about a foot from the instrument, and their light 

 was thrown in at the windows by lenses and reflectors. The re- 

 flectors were fixed on soft wires, so that they could be bent to re- 

 flect a bright light from the bottom of the lines on the studs. All 

 the parts of the instrument were made as heavy as consistent with 

 convenience, to prevent their being affected by sudden changes 

 of temperature. 



The rods of which the expansions were determined varied from 

 half an inch to an inch square, and the length of twenty-three 

 inches was carefully laid off on silver-studs, which were fixed into 

 holes drilled in the rods, the centres of the holes being twenty-three 

 inches apart. It was necessary to divide the head of the upper 

 stud into fiftieths of an inch, in order to determine the value of 

 the micrometer for each experiment, as it was liable to a little 

 variation, from the impossibility of adjusting it to exactly the 

 same focus, which it had when it was adjusted to a fixed value. 

 To guard against this error arising from alteration of the focal 

 distance, I read the micrometer for /„ of an inch on the stud in 

 the rod which was to be heated ; and this was repeated at each 

 experiment made on the rod, after every thing had been adjust- 

 ed. In placing the rod in the instrument a socket was tied to 

 the lower end below the lower stud, and the socket was fitted so 

 as to slip tightly upon the top of a square head, on the end of a 

 pin P, Fig. 3, moved by the lever and screw G, Fig. 1 . This served 

 to raise and depress the rod, and was very convenient in fixing it 

 in the proper place, for the under microscope. In Fig. 3, A is the 

 lower end of the rod, S the under stud in it, B the socket, which 

 is tied to A, by the piece on the front, so that it was of no con- 

 sequence what the thickness of the rod might be, as the stud was 



