34 J. 0. Thompson — Law of Elastic Lengthening. 



the total change in an entire forenoon amounted usually to 

 less than half a Celsius degree. The length of the wires used 

 was about 23 in. 



In order to determine the mean temperature of the tower at 

 any time, as well as the changes in the temperature, I used at 

 first six thermometers hung at regular intervals in the tower. 

 Afterwards at the suggestion of Prof. Kohlrausch I used in 

 place of the six thermometers a fine brass wire 23 m. long, 

 bearing a constant weight, and running parallel with the main- 

 wire, the wire on which the measurements were made. 



Since this thermometer- wire was only 9 cm. distant from the 

 main-wire, and both were equidistant from cathetometer I, I 

 was able, without changing the focus of the telescope, to 

 observe the mark on the thermometer-wire and to determine 

 the temperature accurately to the fortieth of a degree. 



Description of Apparatus. 



In the tower of the Institute are four solid brick piers, three 

 of which terminate within the tower near the top. A large 

 square beam 25 cm. thick was laid across two diagonally oppo- 

 site piers, and this furnished an inflexible support for the 

 upper end of the wire. A smaller beam was laid parallel with 

 the larger, and on this was mounted a microscope focused on. 

 the upper mark on the wire. The larger beam rested on iron 

 feet which were so adjusted that the whole arrangement stood 

 absolutely firm. A movable cross-wire in the eye-piece of the 

 microscope enabled one to measure a displacement of the mark 

 of - 005 mm. Since a weight of 70 kg. on the middle of the 

 beam produced a sinking of only 0*03 mm. and since the 

 heaviest weight used in any of my experiments was 18 kg. we 

 may regard the upper mark as constant. Still to avoid all 

 uncertainty the position of the mark was often noted and found 

 to be unchanged. 



At the lower end of the wire the following arrangement was 

 used : Two cathetometers were mounted on the same stone 

 block, one for the purpose of noting the position of the mark 

 when no weight was on the pan, the other to note the exact 

 point to which the mark was brought by any stretching weight. 

 In order that a measurement might be made quickly, before 

 the after-effect had time to make itself apparent, a damper 

 immersed in glycerine was used. In this way one was enabled 

 to make an accurate setting of the cathetometer within 10 sec. 

 after the weight had been placed on the pan. To render impos- 

 sible any friction between the damper and the sides of the 

 surrounding vessel, the damper was fastend to the upper end 

 of a light rectangular frame, to the lower end of which the pan 



