erical bobs are not used. The great object is
to allow the air ready access to all parts of the rod and compensator,
so that they are all heated or cooled simultaneously. The bobs are
usually made of a compound of lead, antimony, and tin, which forms a
hard metal, free from bubbles and with a specific gravity of about 10.
The usual weight of the bobs of the best pendulums for an ordinary
astronomical clock is about 15 lb. A greater weight than this is found
liable to make the support of the pendulum rock and to put an undue
strain on the parts, without any corresponding advantage. The rods
used are all artificially aged, and have their heat expansion
measured. No adjusting screw at the bottom is provided, the regulation
being done by the addition of weights half way up the rod. An
adjusting screw at the bottom has the disadvantage that it is
impossible to know on which of the threads the rod is really resting;
hence extra compensation may be introduced when not required. It is
considered better that the supports of the bob should be rigid and
invariable.

Barometrical error.

The effect of changes in the pressure of the air as shown by a
barometer is too important to be omitted in the design of a good
clock. But we do not propose to give more than a mere indication of
the principles which govern compensation for this effect, since the
full discussion of the problem would be too protracted. We have seen
that the action of the air in affecting the time of oscillation of a
pendulum depends chiefly on the fact that its buoyancy makes the
pendulum lighter, so that while the mass of the bob which has to be
moved remains the same or nearly the same, the acceleration of gravity
on it has less effect. A volume of air at ordinary temperature and
pressure has, as has been said, .000103 the weight of an equal volume
of type metal, whence it follows that the acceleration of gravity on a
type metal bob in air is .999897 of the acceleration of gravity on the
bob _in vacuo_. If, therefore, we diminish the value of g in the
formula T = [pi]sqrt(L/g) by .000103, we shall have the difference of
time of vibration of a type metal bob in air, as compared with its
time _in vacuo_, and this, by virtue of the principle used when
discussing the increase of time of oscillation due to increased
pendulum lengths, is 1/2(.000103) second in one second, or about 4-1/2
seco