ASSOCIATION UNIT OF RESISTANCE IN ABSOLUTE MEASURE. 
687 
The same method of measurement was applied at the beginning of the third set, and 
a watch was kept by means of the measuring rods already spoken of (p. 667). Slight 
movements were in fact observed, principally of the nature of a recovery of the 
telescope stand from the rather violent treatment to which it had been subjected as a 
test. Minute corrections are accordingly introduced into the tabular statement 
(p. 693), so as to make the results of different days comparable. At the close of the 
spinnings the direct measurement was repeated, when there appeared a slight dis¬ 
crepancy between the results obtained by Mrs. Sedgwick and myself. It is in fact 
rather a difficult matter to say exactly when the pointer has advanced to the equilibrium, 
position of the centre of a suspended mirror, which cannot be prevented from swinging. 
Although the amount in question was not important, I thought it might be more 
satisfactory to check the result by another method, and therefore arranged a travelling 
microscope focussed alternately upon the centre of the mirror and upon a scratch on 
the window of the magnet box, by which the distance between these two points was 
determined. The remaining distance between the scratch and the scale was easily 
measured with the rod. The result tended to confirm the smaller value previously 
found. The value adopted for the spinnings of the third series before November 5 is 
2668*8 millims. 
and for November 5 and subsequent nights 
2669*4 millims. 
From these numbers we have to subtract l’l millim., as a correction for the thickness 
of the glass window; so that 
D before November 5 =2667*7 millims. 
D November 5 and after =2668*3 millims. 
These distances are expressed in terms of the divisions of the scale, whose exact 
agreement with millimetres is of no consequence. 
Reduction of results . 
In order to give a clear idea of the results and of the manner in which they have 
been reduced, it will be advisable to quote from the note book the details of one set of 
spinnings. I have chosen at random one of the third series made on October 31, 1881, 
with a speed of “ 45 teeth.” 
MLCCCLXXXII, 4 t 
