nds in a day of 86,400 seconds. It follows that a barometric
pressure of 30 in. causes a loss of 4-1/2 seconds in the day, equivalent
to .15 second per day for each inch of difference of the barometer.
But, as has already been explained, the effect of the mass of the air
transported with the pendulum must also be taken into account and
therefore the above figures must be doubled or nearly doubled. A
difference of 30 in. of barometric pressure would thus make a
difference of 9 seconds per day in the rate of the pendulum, and the
clock would lose about 1/3 of a second a day for each inch of rise of
the barometer, the result being of the same magnitude as would be
produced by a fall of temperature of 15 deg. F. in the air. Either of
these effects would require a shortening of the pendulum of 1/3000 in.
This estimate is not far from the truth, for observations taken at
various European observatories on various clocks, and collected by
Jakob Hilfiker, give a mean of .15 second of retardation per day per
centimetre of barometric pressure, or .37 second per day for each inch
rise of the barometer.

In order to counteract variations in going which must thus obviously
be produced by variations of barometrical pressure, attempts have been
made purposely to disturb the isochronism of the pendulum, by making
the arcs of vibration abnormally large. Again, the bob has been fitted
with a piece of iron, which is subjected to the attraction of a piece
of magnetized steel floating on the mercury in the open end of a
barometer tube, so that when the barometer falls the attraction is
increased and the pendulum retarded. Again, mercury barometers have
been attached to pendulums. A simple method is to fix an aneroid
barometer with about seven compartments on the pendulum about 5 to 6
in. below the suspension spring, and to attach to the top of it a
suitable weight which is lowered as the barometric pressure increases.
One of the best methods of neutralizing the effects of variations of
barometric pressure is to enclose the whole clock in an air-tight
case, which may either be a large glass cylinder or a square case with
a stout plate-glass front. This renders it independent of outside
variations, whether of temperature or pressure, and keeps the density
of the air inside the case uniform. If the case could be completely,
or almost completely, exhausted of air, and kept so e