Page:Proceedings of the Royal Society of London Vol 60.djvu/122

Rh arranged in descending order of some one meteorological element. Suppose there to be 2 nor 2« + l observations in the series (or half series) ; the mean values of the ratios borne by the electrograph readings to the corresponding ones with the portable electrometer at station A were calculated for the first n and the last n instances separately. Supposing ru r2, and r to denote the mean ratios for the first n, the last n, and the whole 2 n(or 2w + l) observations, then

£{(ri—r2)/r}x 100

may be regarded as the average percentage deviation of the two groups from the mean. Table VIII gives the value of this quantity in the case of the three meteorological elements from which a differential effect was most feared.

Table VII r. Value of \{(ji— )/r} x 100. Series of observations. Times of observations. Meteorological element considered. Yapour density. Sunshine. W ind velocity. I Day - 1 5 .. - 8 TT J Forenoon - 1 2 + 8 + 8 11 i Afternoon + 1 + 4 + 12 i n f Forenoon - 2 0 + 14 + 22 III | Afternoon - 1 5 + 15 - 1 6 IY Day - 1 5 * * < 0 |

A plus sign denotes that when the meteorological element in question was above its mean the water-dropper was more than usually effective, relative to the portable electrometer ; a minus sign implies the contrary.

The evidence in the case of wind velocity is so contradictory that we can safely assume that no uniform differential action exists. In the case of the two other elements the evidence is more consistent, and it is possible that a small differential action may exist. It looks as if much moisture, when not counterbalanced by a contrary action of sunshine, tends slightly to pull down the reading of the waterdropper relatively to that of the portable electrometer. The phenomenon, supposing it to exist, might be ascribed to a loss of efficiency in a water jet when the vapour in the air increases, and a similar loss in a flame collector during bright sunshine. But an influence at least as likely is that of moisture, during damp weather, on the insulation of the electrograph.