xlviii METEOROLOGY OF BEN NEVIS. 



On the other hand, the most marked permanent fall occurs about the autumnal 

 equinox.* 



Hygrometry. — A series of elaborate hygrometric observations was made at the 

 Observatory from August to October, 1885, by Mr H. N. Dickson, under the direc- 

 tion of Professor Tait and Dr Buchan. In this investigation along with the 

 ordinary dry and wet bulb hygrometer, Professor Chrystal's direct hygrometer, in 

 use at the Observatory, was employed. The results obtained are of very consider- 

 able value in determining how far Glaisher's factors, so largely used by meteor- 

 ologists in such inquiries, can be safely used. As regards the remarkably dry states 

 of the air, which form so prominent a feature in the climatology of Ben Nevis, 

 Glaisher's factors were found to be altogether inapplicable, and for such hygro- 

 metric observations a specially constructed set of tables is therefore required.! 



Earth Currents in the Telegraph Cable. — Hourly observations were made by 

 Mr Dickson from 20th September to 4th October 1885. The curve of hourly 

 values, obtained by taking the mean deflection for each hour, showed a well-marked 

 daily variation. During the early part of the night, a current passes through the 

 cable up the hill, and appears to reach a maximum at 9 p.m. and again at 2 a.m. 

 Two hours after a rapid diminution sets in, and at 5 a.m. the current is reversed. 

 Immediately after this the potential at the summit again falls relatively to that at 

 Fort-William, and then a current ascends, increasing in strength to a well-marked 

 maximum at 10 a.m. This is succeeded by another equalisation, and the current 

 is again reversed about 1 p.m., after which it passes down the line till about 6 p.m. 

 Thereafter the current again takes an upward direction through the line, increasing- 

 pretty regularly to the maximum at 9 p.m. These results were confirmed by a 

 series of observations conducted by Mr Omond during the summer of 1888. 



From observations on atmospheric electricity on the summit of Dodabetta, 

 Professor C. Michie Smith has shown that on the edge of a dissolving mist the 

 potential is lower than the normal, but higher on the edge of a condensing mist. 

 Now in nearly every case, when the top of Ben Nevis became clear for a short time, 

 a strong current was coming up the line, while as soon as the summit was again 

 enveloped the current was reversed. The connection between the moisture of the 

 atmosphere and the earth currents is still further shown by the rainfall. During 

 a fall of rain or snow, the current is almost always found to pass down the cable ; 

 and, in the case of a sudden shower, the current sometimes drove the mirror of the 

 galvanometer violently off the scale. A cessation of rain or snow generally had 

 an exactly opposite effect. 



If it be assumed that the summit of Ben Nevis takes the potential of the 

 masses of vapour covering it, and the earth-plate at the base be considered as the 



* " Daily Mean Temperatures at Ben Nevis Observatory and Fort- William," by R. T. Omond, Jour. Scot. 

 Met. Soc, Third Series, vol. iii. 



t " The Hygrometry of Ben Nevis, by H. N. Dickson, Proc. Roy. Soc. Edin., vol. xiii. p. 957. 



