170 G. W. Hough on Cataloguing and Charting Stars. 
_ Now, when the telescope is moved in zenith distance, motion 
is given to the steel pen so that it moves over the cylinder in the 
direction of its axis. Whenever we wish to make a record, a 
key is pressed which closes the circuit through the electro-mag- 
net, and a blow is struck on the arm carrying the steel pen; 80 
that a small dot is made on the sheet of paper covering the 
eylinder. 
It remains now to show how the magnitudes of the stars are 
recorded on the Chart. Various plans were suggested, and I 
finally decided to represent the magnitudes by different colors. 
For this purpose we use prepared paper, known as duplicating 
impression paper. * 
If a strip of this paper be laid over a sheet of ordinary wil 
ting paper, and a pen be drawn over it, a colored impression will 
be left on the paper. In the same manner, if a blow be str 
with a blunt point, a colored dot will be the result. Now, if at 
the time of observation an assistant should introduce a strip of 
Various kinds of apparatus might be employed to take the 
place of the assistant; but 
tainty. We at first placed 
our strips on a belt running over 6 
what we need is simplicity and cer 
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