FOR DETERMINING THE MEAN DENSITY OF THE EARTH. 
303 
planes, but in the knife-edges. Mr. Simms on examination found that, when the 
attaching screws were relaxed, the bearing on the agate-planes was continuous and 
perfect. It was evident therefore that the fault was in the surface of the brass 
blocks which carry the knife-edges. On filing these it was found easy to bend the 
knife-edge into any form. A surface was at length given to the brass which made 
the bearing of the knife-edges upon the agates absolutely perfect, as far as the eye 
could discover. 
The upper and lower pendulums, when mounted in their proper stations, vibrated 
in parallel planes, as nearly as possible in the direction of magnetic East and West. 
The graduated arcs for measuring the extent of vibration were divided to inches, 
with continuous numeration from one end ; and were placed behind the pendulum 
tails. 
The thermometers (two for each pendulum) were suspended in front, at about ^ 
and f the distances from the knife-edge to the bob. Their indications were found to 
be all sensibly accordant. 
17 . In fitting up the comparison-clocks, a small alteration was made which proved 
exceedingly convenient. The illuminated disk (to be concealed by the tail of the 
invariable pendulum) was an inclined section of a small cylindrical block attached 
by a central screw to the bob of the clock pendulum. The inclined surface was 
covered with gold-leaf. A hole (covered with glass) was made in each side of the 
clock-case, and through either of these the light of the illuminating lamp was thrown 
upon the gold-leaf. By shifting the lamp and turning the small cylindrical block, a 
brilliant light was reflected to the observing-telescope, the lamp being always in 
a distant position and in a lateral direction. Moreover, by slightly turning the clock 
stand in azimuth, the apparent breadth of the inclined disk was altered, and it could 
thus be adapted to disappearance behind the pendulum-tail. 
The adjustible aperture through which the disk was seen, and which was covered 
by the pendulum-tail in its quiescent position, was in front of the clock-case. 
18. By the side of each clock-face a galvanometer was fixed. The galvanic wires 
were led to and from the terminals of the galvanometer, not immediately, but through 
the intermediation of a circuit-breaker; so that the observer could at any time inter- 
rupt the current. 
19. The journeyman-clock was thus fitted up. Two wires were led into it (one 
from the galvanic battery, and the other in continuation of the course of the same 
wire from the journeyman to the next comparison-clock), terminating within the 
journeyman in a pair of springs which performed the duty of circuit-breaker. Upon 
the minute-wheel of the journeyman were four pins, which, as the wheel revolved, 
pressed the two springs together, thus completing the circuit (in that part) at every 
15® of the journeyman’s time. 
20. The battery was the ordinary sand-battery, of 24 cells. The batteries and 
journeyman were in the side-room of the upper station. 
