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84 A. M. Mayer—LEffects of Magnetization 
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effect in disturbing the scale-reading 
To ascertain whether the mirror accurately followed the 
changes in length of the rod, I repeatedly made the following 
observation. The readings of the screw-head and the scale were 
—— ; then the screw was rotated by an assistant, so as to pus 
efore it the rod. The scale-readings ran up steadily with the 
rotation, and when the screw was rotated backward the scale- 
readings ran smoothly down, and when the screw-head had 
reached the same position 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 centl- 
meter, corresponded to a change of ‘00011 inch in the length of 
the rod ; but $ millimeter, or ;'; of a division, could be precise 
ly read in the telescope; I am therefore justified in believing 
that the measures I will give in this memoir can be relied on t0 
the s3¢ss7 Of an inch. 
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, tele 
Mr. Saxton, 1 turn=0°01912 inch 
As the result of numerous determinations, made on various 
parts of the screw, I found that ;1, of a rotation of the screw 
equaled 1-737 divisions of the telescope scale ; which tie 
000011 inch as the value of one division of the scale. 
value, however, applies only to the experiments on the rods of 
