the Pendulum vibrating Seconds in the Latitude of London. 95 
ated in a part of Portland Place not liable to much disturbance 
from the passing of carriages. The room in which the experi- 
* ments were made is the last of two on the ground floor, commu- 
nicating with each other and facing the north. The tempera- 
ture consequently is very steady, and, if necessary, may be raised 
to any given degree by a fire in the frst room. The clock with 
which the pendulum was compared was made by Arnold; and 
in addition to the gridiron compensation for temperature, its 
pendulum is suspended by a spring, the strength of which is so 
adjusted, that the vibrations in different arcs are performed in 
equal times. This clock is firmly screwed to the wall, in a re- 
cess opposite to the window. Near to this, on the wall which 
is at right angles to the recess, is fixed another time-piece by 
Cumming, which was the property of the late General Roy, and 
is considered by Mr. Browne to be the best in his possession. 
Respecting this clock, it will be sufficient to remark, that three- 
tenths of a second was the greatest variation in its daily rate 
from the 22d February, when the observations commenced, to 
the 31st July; and consequently the deviation from its mean 
rate during that period, did not exceed 0°15 of a second per day. 
This clock has been used as the standard of comparison, the 
time having been taken from the transit instrument by a chro- 
nometer.of Arnold’s. With such advantages it will be confessed 
that there can be little chance of error arising from the rate of 
the clock. 
A plank of well seasoned mahogany, two feet wide, and three 
inches thick, was forcibly driven between the walls forming the 
sides of the recess, until it was near the top of the clock case. 
Yo this the support of the pendulum before described was firmly 
screwed, and carefully levelled, in such a position as to allow the 
pendulum to vibrate as near as possible to the clock-case with- 
out touching it; and that, when at rest, it might appear to an 
observer in front of the clock, to pass over the centre of the dial- 
plate, its extremity reaching a little below the centre of the ball 
of the pendulum. Beneath, fixed to the clock-case, was an arc’ 
divided into degrees and tenths, to determine the extent of the 
vibrations. Such a portion of the plank was cut away as wes 
necessary to admit of the pendulum being placed on its support. 
A cireular white disk was pasted on a piece of black paper, which 
was attached to the ball of the pendulum of the clock ; and this 
disk was of such a diameter, as, when both pendulums were a¢ 
rest, to be just hid from an observer at the opposite side of the 
room, by one of the slips of deal which form the extremities of 
the brass pendulum, 
Though there was little reason to imagine that the vibrations 
of the pendulum could communicate any motion to a support so 
firm 
