'rhe zinc tube passed through a hole in the bob to its centre, 
so that the expansion or contraction of the zinc tube raised or 
lowered the centre of the bob, thereby avoiding the irregu- 
larities which arise from the expansion of the bob itself in 
other pendulums. As the bob is a large mass of metal, a 
change of temperature (which almost immediately expands or 
contracts the rod and the compensating tube) does not per- 
meate it for several hours ; but when suspended by its centre 
it is obvious that the change of length which takes place 
when it has arrived at its new temperature, does not aifect 
the rate of the clock, because it expands or contracts equally 
above and below its centre. If, therefore, the lengths of the 
pendulum rod and the compensating tube are correctly 
adjusted, the centre of the bob is kept constantly at the same 
distance from the point of suspension. The bob used in 
this pendulum, and the one described hereafter, are neither 
cylindrical nor lenticular shaped. They are about ten inches 
long and four wide, with straight sides, and a transverse section 
would resemble a section of a lenticular bob ; thus, the sides 
terminating in rather sharp edges, very little disturbance is 
caused in the air confined in the clock case. 
Although pendulums made as above described have been 
tried and found to answer very well in dwelling-houses, they 
do not appear so well adapted to clocks in observatories. 
They have been tried at Mr. E. H. Greg’s Observatory, at 
Quarry Bank, and at Mr. Robert Worthington’s, at Crump- 
sail, and irregularities were observed which seemed to arise 
from other causes than imperfect compensation. Although 
the wooden rods were carefully selected from dry and straight- 
grained deal, and protected from moisture by varnish, a 
change in the condition of the atmosphere, from moist to dry 
and vice versa, was generally followed by a change of rate in 
the clock. 
Mr. Lowe then determined to use steel instead of wood for 
the rods ; and as a much greater length of zinc would be 
