NEWTONIAN CONSTANT OF GRAVITATION. 49 
held down for one second, which produces a “transit mark,” the purpose of which is 
to indicate that the previous dash was made at a transit of the pointer. Immediately 
afterwards I note in the book an arrow showing the direction of motion. Every 
transit is thus marked during the first stage, which lasts 10 or 15 minutes. In order 
to know the actual time of any of the minute marks, I hold the key down once ata 
minute for six seconds and thus produce “time mark.” This also is noted in its 
proper place in the book with the time. To still further make sure of the place in 
the book which corresponds to any place on the smoked drum, I occasionally make 
a “castle mark,” that is, hold the key down during alternate seconds four or five 
times, and note that also in the book. The proper sequence of time marks, castle 
marks, and transit marks, is sufficient to make it evident afterwards to which arrow 
in the book any particular transit mark belongs. During this stage it is net possible 
to take the elongations as they are off the scale. I then leave the apparatus to itself 
for about 20 minutes after first stopping the drum. On my return | start the drum, 
secure another time mark, and every transit, as before, with a castle mark soinewhere 
for distinction, but now the elongations being observable I note them in the book at 
the same time. Following the practice of Professor Cornu, but on an extended 
scale, I take the transits of the chief divisions at first of every 1000 divisions, then 
of every 100, and after a time of every 10. These are distinguished on the drum by 
four rapidly repeated dashes after a 1000, three after a 500, two after a 100, one after 
a 50, and none aftera 10. I cannot take single divisions as they pass by so rapidly. 
I have not used these marks except in rare instances, but they are available for 
reduction in case time for the very tedious calculation could be found. Fig. C is 
a full size reproduction of a portion of the sheet of October Ist, 1893, after L had gone 
over it and scratched in the actual times. The different classes of marks are all to be 
seen. I do not, as a rule, find it possible to put in arrows while writing elongations, 
and marking transits of divisions as well as of the pointer. Their existence 1s 
understood between elongations. In addition, I generally place a letter or word 
to distinguish good observations of transits from bad, thus: — ov atl, 
or — 04 late. I do not think the v.g. observations can be more than ‘01 second 
in error, or the g. more than ‘02 or ‘03 second. Those unmarked might be perhaps 
as much as ‘1 second, though they may also be good, but those marked bad would 
probably be more. If it were not for the high period tremor set up by the trains, 
which prevent good observations when the amplitude is less than about 40 divisions, 
I should expect to obtain a higher degree of accuracy in the periods than are actually 
obtainable. I sometimes take observations of deflection and period on more than one 
night. 
Operation 11. 
Transfer the gold balls to the side hooks, and leave for a day, if possible, to quiet. 
Find the deflection, if any, produced upon the mirror alone, by moving the lid and 
MDCCCXCV.—A. H 
