In which β is the impulsive arc and α the definite arc, showing that α comes out nearly equal to 1/2 β. 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°, with the temperature of the atmosphere at 20° C., and with the difference of 4° to 5° 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 tension I took that which corresponded to a single pair, that is, the thirtieth part of the total tension. Other electrometers may be compared with one already properly adjusted, without always having recourse to the pile. This done, let us see the arrangement of all the apparatus: _H H_ (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty room, with an opening, _o o_, at the upper part. _M M_, a bracket or table fastened against the wall, about a metre distant from the ceiling, _H H_. _N N_, a wooden platform for the observer. _A_, the bifilar electrometer. _B_, Bohnenberger's electroscope.