the following
equation:

A (B - A) / B = tang. (1/2) A

In which B is the impulsive arc and A the definite arc, showing that A
comes out nearly equal to 1/2 B. In dry weather all goes perfectly
within the limits of proportion, and I can tell whether, during the time
in which the index traversed the impulsive arc, there were any
_dispersions_ and of what nature; for if the definite arc is not close
to the limit of the impulsive arc, it is a sign of _dispersions_ having
taken place during the motions of the index. Every degree less in the
definite arc denotes two degrees of loss for the impulsive arc; but as
the index employs double the time traversing the definite as it does the
impulsive arc, we may consider the loss of one equal to the loss of the
other.

In excessively damp weather the index gives no definite arc, and it is
necessary to resort to artificial heat in order to dry the insulators.
The most simple means I know of is to hold the instrument over some
hollow vessel, which, for the time, is converted into a stove by the
introduction of a spirit lamp.

From Gauss's formula for the bifilar system of instruments of this
class, we learn that the maximum sensitiveness of such instruments is
given when the length of the suspending fibres is greatest, and the
distance between them is smallest, with the weight of the movable or
rotating member a minimum; and these elements being the same, the
sensitiveness of the instruments is invariable.

To some electrometers, in order to avoid errors of parallax, a small
telescope, with a micrometer wire, has been added; but, with a little
practice, we can read accurately without this refinement. In order to
obtain comparative measurements, it is necessary to select some given
unit of tension. I have observed that by making a galvanic pile of
copper, zinc and distilled water, and insulating it well, each pole has
a tension which remains the same for many days, if the conditions of
temperature and the moisture of the surrounding atmosphere are not very
different. With thirty pairs of this pile, each element having
twenty-five square centimetres of surface, I have on the electrometer a
definite arc of 15 deg., with the temperature of the atmosphere at 20
deg. C., and with the difference of 4 deg. to 5 deg. C. between the
thermometers of the psychrometer of August's construction. The first
observation was made twenty-four hours after mounting the pile. For unit
of tensio