PROCEEDINGS OF SECTION E. 281 
at the telescope can see towards the reversion pendulum. The 
distance of this hole — the centre of the telescope is 8 it. 
3.75 in. 
It remains yet to be mentioned that, as I had entertained 
serious apprehensions for the safety of the clock in case the 
moisture of the room might cause it to rust, I made arrange- 
ments for placing basins filled with chloride of lime near the 
clockstand for the purpose of drying the air surrounding it. 
A thermometer inside the clock is registered to indicate the 
degree of compensation achieved by its mercurial pendulum. 
THE REVERSION PENDULUM. 
There can exist no necessity for entering more fully into 
a description of this instrument, as in its main features it 
corresponds with that of Geniain Kater, and only the follow- 
ing weights must be given: 
"The total weight of the pendulum, without bobs, is equal to 
64,530 erains, which weight is symmetrically arranged on both 
sides of it proceeding from the centre of the central weight 
of 12,960 grains, already included in the above total weight. 
With a view of basing thereon a computation of the centre of 
gravity, as will be mentioned hereafter, the weights of single 
parts, which could not be placed on a balance, have been com- 
puted, but I refrain from giving here all particulars, and I 
mention only that the specific gravity of the metal of which 
this instrument is made was found to be 8.3. Including the 
two bobs of the respective weights of 16,270.5 and 2976 grains, 
the whole weighs 83,776.5 grains. All measurements have been 
learnt from the diagrams representing the apparatus, which 
were carefully prepared by Mr. Straubel, so much only may 
be mentioned, that the exact distance between the two knife- 
edges is equal to 39.368 in. at a temperature of 54.5 deg. Fahr. 
The instrument has been constructed according to the sug- 
gestion thrown out by Prof. Bessel in his treatise on the length 
of the pendulum vibrating seconds at Koenigsberg (Unter- 
suchungen tber das einfache. Secundenpendel, chapter 31, 
p- 96), and the points in which it differs from that of Captain 
Kater, described in the Philosophical Transactions of 1818, are 
these :— 
(a) For the purpose of compensating the buoyancy of air 
in the oscillations, two cylinders are used of exactly the same 
dimensions, and placed in exactly the same position with regard 
to the centre of the central weight. It is proved that by such 
an arrangement the oscillations are independent of the resis- 
tance of the air (see Bessel, p. 95); the one of these cylinders 
is massive, the other hollow; the respective weights are already 
given above. 
