in changing the dimensions of Iron and Steel bars. 88 
tered in the helix h. This helix is 60-25 inches long, and has 
the rods. The deflections of the mirror, caused by the elonga- 
tions or retractions of the rods, were determined by means of 
the telescope and the scale, represented at 1. The telescope and 
scale were placed between 5 and 6 meters from the mirror, and 
the scale was divided into millimeters. Each unit of the di- 
vision given in the experiments corresponds to one centimeter of 
the scale. The above apparatus was placed in a cellar room 
. 
. 
without windows and entirely under ground; and the room 
Was always entered by a door, the bottom of which was ona 
level with the ceiling of the room. During the course of the 
Enea lg the range of variation of temperature of this room 
did not exceed 0-1° C. in twenty-four hours. 
Examination of the stability and degree of precision of the 
apparatus. 
A rod was accurately centered in the helix, and the micro- 
meter screw and the slide of the mirror were brought in con- 
fact with its ends. After the heat imparted to the rod and 
apparatus by handling had been dissipated, and the scale-read- 
ing had remained constant for an hour, I pushed the mirror- 
slide away from the rod and then again allowed it to come 
