164 Captain Sabine’s experiments to determine the 
heads of the screws securing it, were sunk below the surface, 
and concealed by brass pins, to prevent their being removed : 
the knife edge was carefully adjusted, so as to be at right 
angles to the direction of gravity: a very firm support of 
brass was screwed to the thick plank which forms the back 
of the clock case; in this were imbedded two pieces of 
agate, which were ground into portions of hollow cylinders, 
finished in their places to receive the knife edge of the pen- 
dulum : parallel to the agates a small level was fixed in the 
direction of the cylinders, by means of which they could be 
placed truly horizontal : an arc divided into degrees and 
tenths, but which might be read off by estimation to hun- 
dredths, was attached to the back of the clock case at the 
bottom of the pendulum, to give the arc of vibration. 
“ Each clock was furnished with a triangular support of 
wood contrived by Dr. Wollaston, and so firmly arranged, 
that there appears no reason to apprehend any motion in the 
point of suspension ; and it is sufficiently obvious that no 
change can take place in the length of the pendulum, but 
such as may arise from a variation in temperature. For the 
purpose of distinction, the two clocks and the two pendulums, 
are marked respectively No. 1 and No. 2.” 
To this description it may be added, that the clocks were 
cleaned and oiled by Mr. Arnold, in March 1818, previous 
to the commencement of the experiments, and that these 
operations have not been needed subsequently. The oil 
which was used was prepared by Dr. Wollaston. The 
preparation consisted in submitting it to a low tempera- 
ture, when the part which remained fluid was separated by 
a gentle pressure from that which had become solid ; the 
