246 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
a slit was made in the cork to allow it to be fixed upon that arbor. Through the cork 
a thick copper wire was passed stretching horizontally for 3 inches, where it was bent 
downwards at a right angle. This bent arm was half an inch long, was amalgamated, 
and dipped, at each oscillation of the pendulum, into a cup of mercury connected with 
the electric circuit. The stout copper wire was also connected with that circuit by 
means of a very fine copper wire. This arrangement was very perfect. 
The chronograph .—In all the experiments previous to 1880 we used one of the 
ingenious portable chronographs constructed by M. Hypp, of Neuchatel. In this 
instrument a strip of paper is run off from a drum, and upon it the signals from the 
clock and wheel are inscribed by two siphon pens. Uniformity of motion is given by 
the mechanism of the chronograph giving motion to a spur wheel against which the 
end of a spring presses. This spring permits one, two, &c., teeth of the wheel to pass 
in the course of a single vibration, the number allowed to pass being under control of 
the experimenter. A large number of observations were made with this apparatus. 
But we came to the decided opinion that while it is admirably adapted for observatory 
work where a greater accuracy than ^th second is not required, yet in order to get 
the most perfect possible results a different class of instrument must be employed. 
Consequently we designed a - new chronograph, which was constructed for us by 
Messrs. Elliott Brothers. The principle of this apparatus is that we depend for 
uniformity of motion on the inertia of the apparatus. It was our object to get rid of 
all clockwork, and by making use of a fly-wheel, which has no work to do, to get rid 
of a host of irregularities which affect (it may be in a very slight degree) all other 
chronographs. 
The base of the instrument consists of two strong triangular castings bound together 
in a horizontal position by three brass pillars. Between these triangles a fly-wheel, 
12 inches in diameter, rotates on a vertical axis. The lower end of this axis is a hard 
steel socket which rests upon the point of a strong screw working in the centre of the 
lower triangle and firmly fixed there by means of a nut. 
The lower triangle is supported upon three feet adjustable for levelling by screws. To 
the upper triangle two vertical brass pillars, 16 inches long, are fixed. They are 
connected at the top by a cross piece of iron, in the middle of which a thick pin works 
vertically with a screw motion, and it can be fixed by a nut. 
The lower end of this pin terminates in a point, supporting the hard steel socket at 
the upper part of the axis of the registering cylinder. This cylinder is of brass. It 
is 12 inches long and 4 inches diameter. Its axis projects an inch at each end. The 
upper end, as before said, rests against the point of the upper screw. The lower end 
is, during an observation, firmly fixed to the axis of the fly-wheel by means of a solid 
brass tube closely fitting these two axes, with six screw nails to ensure rigidity. 
Bound the upper end of the axis a silk thread is wound which, passing over a pulley 
mounted on friction wheels, carries a small weight. The axis of the cylinder is made 
vertical by the levelling screws, a spirit-level being placed on the top of the cylinder 
