304 
MR. AIRY’S ACCOUNT OF THE BARTON EXPERIMENT 
The wire from one end of the battery was led to one terminal of the journeyman. 
From the other terminal of the journeyman a wire was led to the circuit-breaker in 
connexion with the galvanometer of the upper clock. From the other terminal of 
the galvanometer a wire was led down the mine-shaft to the circuit-breaker in con- 
nexion with the galvanometer of the lower clock. From the other terminal of this 
lower galvanometer a wire was led up the mine-shaft to the other pole of the battery. 
Thus when both the upper and the lower circuit-breakers completed contact (and at 
no other time), the journeyman-clock made the circuit complete and sent a current 
through both the galvanometers at every 15® of the journeyman’s time. 
20*. Plate XI. contains views of the pendulum-apparatus nearly in the state in 
which it was used in the upper station. The principal diagram is a front view of 
the apparatus as mounted at Greenwich, taken with the camera lucida, and may be 
trusted for general accuracy. The iron bars of the pendulum-stand are about inch 
square. The stand of the clock does not touch the pendulum-stand in any part. The 
hexagonal frame introduced into the box-frame is conspicuous in this view. The 
battery is not the same which was used at the mine. The mounting at the lower 
station was exactly similar, wanting only the journeyman-clock and the battery. 
The frame with agate-planes (represented on a larger scale), which is planted on the 
top of the box-frame, is supported on three screw-feet : the screw-stalks are per- 
forated ; two are cut with an internal screw-thread, and long screws are passed 
upwards through smooth holes in the box-frame, and act in these internal screw- 
threads and draw them firmly down: the perforation in the third is smooth, and the 
long screw which passes down through it acts in a screw-thread cut in the box-frame. 
At the sides of the blocks which carry the agate-planes are notched brass plates turn- 
ing upon pins, connected by a stouter piece of brass beyond the pins, through which 
a screw passes that acts in the solid block below; by driving this screw the notches 
are raised, and engage with the ends of the pendulum-knife-edge, and lift it off the 
agates. 
21. The system of observations which I proposed was the following. One of the 
invariable pendulums was to be mounted at the upper station and the other at the 
lower station, and the two pendulums were to be observed simultaneously by two ob- 
servers. The “Swings” or series of vibrations were to follow each other incessantly, 
day and night, with no more interruption than would be required for observing the 
galvanic signals, reading thermometers, and making petty adjustments. Six Swings 
(each occupying in the gross four hours) were to be taken in each day. At con- 
certed minutes of time before the first and after the last Swing, and between the end 
of each preceding Swing and the beginning of that which followed, galvanic signals 
were to be observed. Several “Coincidences” of the vibrations of the detached 
pendulum with the clock pendulum were to be observed in Rater’s manner at the 
beginning and end of each Swing, This system was to be maintained during the 
whole efficient time of one week : then the pendulums with their agate-planes and 
