182 REPORT—1890. 
having an annual period just the reverse of what obtains in the eastern 
districts of Scotland. During the summer they are twice as frequent 
at Fort William as at the observatory, thus suggesting that a consider- 
able number must be below the summit, or in the aerial stratum between 
the high and low level observatories. All the summer thunderstorms 
have occurred when the sun was above the horizon; but of the thirty- 
seven cases in autumn and winter thirty-two took place when the sun 
was below the horizon. These results are of great value in their relation 
to the distribution of thunderstorms and other electrical displays over the 
land and the water surfaces of the globe. 
An elaborate series of hygrometric observations have been made at 
the observatory with the view of inquiring how far Glaisher’s factors can 
be safely used. For the conduct of such an inquiry, the low-temperature 
humidities and remarkably dry states of the air which form so prominent 
a feature in the climatology of Ben Nevis, the observatory offers unique 
facilities. The observations were made with the ordinary dry and wet 
bulb hygrometer and Professor Chrystal’s direct hygrometer, with the 
result that a specially constructed set of tables is required for the extremely 
low humidities of Ben Nevis, these being considerabiy lower than Mr. 
Glaisher had had an opportunity of observing. 
Professor C. Michie Smith has shown that on the edge of a dissolvirg 
mist the potential is lower than the normal, but higher on the edge of a 
condensing mist. Now, almost always when the top of Ben Nevis be- 
comes clear for a short time, a strong current comes up the telegraph cable, 
while as soon as the summit is again enveloped the current is 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 nearly always passes down the cable; and in the case of a sudden 
shower the current has sometimes driven the mirror of the galvanometer 
violently off the scale. A cessation of the rain or snow generally has an 
exactly opposite effect. Ifit be assumed that the summit of Ben Nevis — 
takes the potential of the masses of vapour covering it, and if we consider 
the earth-plate at the base as the earth, or zero of potential, it is obvious 
that these results confirm the theory advanced by Professor Michie Smith, 
a conclusive proof of which would be of the greatest importance in inves- 
tigations connected with thunderstorms. 
Observations on the numbers of dust particles with the apparatus in- 
vented by Mr. Aitken have recently been undertaken at the observatory. 
Already noteworthy results have been obtained. On March 31, at 4.30 P.m., 
the summit was clear, and the number of particles per cubic centimétre 
was 2,785; but shortly thereafter a thickness was observed approaching 
from south-west, which by 6 p.m. reached the observatory, and the num- 
ber of particles rose to 12,862, being the maximum yet observed. On 
June 15 many observations were made during the day, when the number 
of particles fell from 937 at midnight to 50 at 10.30 and 11.42 a.m. Still 
more remarkable were the observations of July 20-21. Tull 10 p.m, of 
the 20th the wind at the top of the mountain was about the direction as 
at sea-level, viz., south-west to west-south-west; but at that hour it went 
suddenly round to north, increasing at the same time to 40 miles an hour, 
and temperature rose from 41°°0 to 47°-0, and soon after to 49°°2. At the 
low-level observatory temperature remained exceptionally constant at 
55°°0 from 9 p.m. till 4 A.M. of the 21st. At the high-level observatory | 
ten observations made between 2 and 3 A.M. gave the extraordinary low 
