Page:Proceedings of the Royal Society of London Vol 60.djvu/115

100 have the influence of a massive stone pillar some 2 | square feet iu section. A calculation of the potential gradient which regards the observations at A as referring to a spot 60 inches above the ground in the open is certain to give an under-estimate. As it is impossible, however, to dispense with a support of some kind, and the presence of the observer is also a disturbing influence, no exact allowance can be made for this.

There have been four principal series of observations. In the first, occupying part of November and December, 1894, observations were taken, when practicable, once a day at stations A, B, C, D, and latterly at E also. In the second series, during part of March and April, 1895, observations were usually taken about 10.30 a.m. and 4.30 p.m. at each of the stations except E. The third series, during part of June aud July, 1895, closely resembled the second; and the only material difference in the fourth was the substitution of station P for station D.

No observations wrere taken on Sundays or on Saturday afternoons. The observations were taken in a fixed order, and, thanks to the skill of the observer, Mr. E. G. Constable, a complete set of readings occupied only some seven or eight minutes. The time scale of the electrograph curves is far from open, and for this and other reasons I have judged it best not to attempt to reduce the readings with the portable electrometer to a common instant.

§ 5. I have taken A as base station, and have found the ratios borne to the individual readings there by the corresponding readings at the other stations.

Let rA, rB represent corresponding readings at A and B, and let L va = \s ( /bA a), 71 where 2 denotes summation for a series of n observations. Then rB/A may be called the mean value of the ratio for the series of observations.

Also let us apply the term percentage deviation of the ratio from its mean to the quantity w r B/A in which the terms in the numerator are taken irrespective of sign.

Table II gives the extreme and mean values of the ratios during each series of observations, excluding three or four occasions when negative potentials were met with.