180 Prof. A. M. Mayer on the Magnetic Elongations 



handling had been dissipated, and the scale-reading had remained 

 constant for an hour, I pushed the mirror-slide away from the 

 rod and then again allowed it to come against its agate terminal. 

 This operation was performed as expeditiously as was consistent 

 with the careful avoidance of shocks to the apparatus. The 

 thermometers, which were placed near various parts of the ap- 

 paratus, were now read ; and it was ascertained that they gave 

 the same readings as before the apparatus was touched. The 

 scale was now read in the telescope ; and it was ascertained that 

 the cross-threads bisected the same division observed before the 

 slide was moved, thus showing that the mirror returned to the 

 position it had before its rotation. This important fact was re- 

 peatedly confirmed with all the rods and at various stages of the 

 research. Indeed the measures of hundreds of experiments 

 made on the elongations and retractions of rods also confirmed 

 the confidence I obtained in the indications of the apparatus 

 arrived at by the preceding experiments. 



Jumping on the floor of the room and the passage of carriages 

 and carts in the streets had not the slightest effect in disturbing 

 the scale-readings. 



To ascertain if the mirror accurately followed the changes in 

 length of the rod, I repeatedly made the following observations. 

 The readings of the screw-head and the scale were noted ; then 

 the screw was rotated by an assistant so as to push before it the 

 rod. The scale-readings ran up steadily with the rotation ; and 

 when the screw was rotated backward the scale-readings smoothly 

 ran down ; and when the screw-head had reached the same posi- 

 tion it had before it was touched, I found that the scale-reading 

 corresponded to that noted when the screw previously had this 

 position. This observation, repeatedly made, gave me the means 

 of testing the precision of the instrument during the progress of 

 the investigations. 



It will be seen below that one division of the scale, or 1 cen- 

 timetre, corresponded to a change of *0001 1 inch in the length 

 of the rod ; but ^ millimetre, or ^ of a division, could be pre- 

 cisely read in the telescope; I am therefore justified in believing 

 that the measures I shall give in this memoir can be relied on to 

 the nn } n ■ of an inch. 



2 



Determination of the value of one division of the scale in changes 

 of length, in inch-measure, of the rods. 

 Pasted on the inside of the box in which the mirror, telescope, 

 and micrometer-screw came to me was the following : — " Abut- 

 ting-screw of field pyrometer. By seven comparisons of five 

 turns with 0*1 inch on Troughton-scale, in June 1857, by Mr. 

 Saxton, jl turn = 0'01912 inch." 



