value the charge remains constant, but as soon as we reduce the time below this value the charge diminishes. The value of T when the diminution in the field begins is T2, the time taken for a positive ion to cross from A to B under the electric field; thus from T2 we can calculate the velocity of the positive ion in this field. If we still further diminish T, we shall find that we reach a value when the diminution of the positive charge on B with the time suddenly becomes much more rapid; this change occurs when T falls below T1 the time taken for the negative ions to go from one plate to the other, for now when the field is reversed there are still some negative ions left between the plates, and these will be driven against B and rob it of some of the positive charge it had acquired before the field was reversed. By observing the time when the increase in the rate of diminution of the positive charge with the time suddenly sets in we can determine T1, and hence the velocity of the negative ions. The velocity of the ions produced by the discharge of electricity from a fine point was determined by Chattock by an entirely different method. In this case the electric field is so strong and the velocity of the ion so great that the preceding methods are not applicable. Suppose P represents a vertical needle discharging electricity into air, consider the force acting on the ions included between two horizontal planes A, B. If P is the density of the electrification, and Z the vertical component of the electric intensity, F the resultant force on the ions between A and B is vertical and equal to _ _ _ / / / | | | Z[rho]dxdydz. _/_/_/ Let us suppose that the velocity of the ion is proportional to the electric intensity, so that if w is the vertical velocity of the ions, which are supposed all to be of one sign, w = RZ. Substituting this value of Z, the vertical force on the ions between A and B is equal to _ _ _ 1 / / / - | | | w[rho]dxdydz. R _/_/_/ But [integral][integral]w[rho]dxdy = [iota], where [iota] is the current streaming from the point. This current, which can be easily measured by putting a galvanometer in series with the discharging point, is independent of z, the vertical distance of a plane between A and B below the charging point. Hence we have _ [iota] / [iota] F = ------ | dz = ------·z. R _/ R This force must be counterbalanced by the difference of gaseous pressures over the planes A and B; hence if pB and pA denote respectively the pressures over B and A, we have [iota] pB - pA = ------ z. R Hence by the measurement of these pressures we can determine R, and hence the velocity with which an ion moves under a given electric intensity.