94 PROFESSOR CG. V. BOYS ON THE 
Sevres, but up to the present I am quite content with that made upon the Whitworth 
machine. 
The Clock. 
The clock, the position of which is indicated in fig. 18, is a Frodsham regulator, 
which was lent to me by the late Professor PrrrcHarD, who took a great interest in 
the experiment. The present owner has kindly allowed me to retain it until the 
work is finished. The clock is placed so that it can be seen from the observing stool 
at the telescope, and is illuminated when necessary by a small incandescent lamp. 
It is employed to mark time upon a smoked drum, upon which also are marks made 
by the action of a key at the telescope. I finally determined to employ the chrono- 
graphic method after seeing Professor Cornu’s apparatus. To the lower end of the 
pendulum I screwed a platinum wire flattened and filed to a rounded edge at its 
end, the edge being in the plane of oscillation. This passes through a horizontal 
line of mercury, standing up by its capillarity above a transverse groove in a piece of 
wood. The end of the groove opens into a large well filled with mercury, so as to 
retain the purity and the level. The wood on either side of the groove is cut away 
to an edge, so that mercury dust carried over by the platinum cannot accumulate 
and give trouble. The wood is so placed that when the pendulum is moving through 
a small swing of a quarter of an inch only, the time marker actuated by the contact 
ticks regularly. With the full excursion the alternate marked seconds are then 
indistinguishable in length. I soldered two platinum wires to the second hand and 
brought an insulated elastic platinum point over the seconds dial and under the 
minute hand, so that the second hand should make contact twice a minute. I so 
bent the wires that at the minute contact should be made again immediately after 
the pendulum had broken contact, and retained till the end of the first second, while 
at the half-minute it was made again after the thirtieth second and immediately 
broken. The minutes and the half-minutes were in this way clearly and differently 
marked (fig. C, p. 48), and it was thus unnecessary to count more than 15 seconds on 
the charts. The time markers and the drum to carry the smoked paper were made 
by the Cambridge Scientific Instrument Company, and are of their well-known 
pattern. I took out the long heavy wires of the time markers, which, I understand, 
are made to the order of the physiologists, and replaced them by very small and 
light styles of copper foil tapering to a point. The two are arranged close together 
so that the tracing points are not more than 7}, of an inch apart. The sheets of 
paper, 12 X 19 inches, are most readily smoked when stretched on the drum by 
pouring a little benzine into the india-rubber pipe which supplies a fishtail burner. 
The very smoky gas flame which results rapidly produces a deep and uniform coat of 
soot. The sheets when finished are passed through a bath of very dilute shellac varnish, 
the strength being such that the smoke does not rub off, but may have numbers, &c., 
readily scratched upon it with a pointed style. The drum is driven through worm 
