180 REPORT—1 890. 
rises Just as much above the normal difference as it falls below the same 
difference in exceptionally dry months. 
At the observatory at the top the annual number of fair days, or days 
when the rainfall is less than the hundredth of an inch, on the mean of 
the six years 1884-89, is 100, the monthly mean rising to the maximum — 
of 12 days in April and June, and falling to the minimum of 5 days in 
July. For any separate month the greatest number of dry days was 20 
in August 1885, whereas in July 1886 no dry day occurred at all. 
The sunshine record extends from March 1884 to the end of 1889. 
The results show an annual mean of 719 hours’ sunshine against a possible 
4,470 hours. Thus, during these six years, the hours of sunshine shown 
by the Campbell-Stokes sunshine-recorder have been nearly one-sixth of 
the number possible. The mean monthly maximum is 149 in June, and 
minimum 19 in December. In December the number has been persis- | 
tently low, even the highest being only 28 hours in 1887. On the other | 
hand, in June the number has exceeded 200 in each of the last three | 
years, rising to 250 hours in 1888; whereas the highest number for any 
of the other eleven months was only 162 hours in July 1885. As will be | 
seen from Table II. the differences between the maximum and the mini. | 
mum numbers of the months are very great. For each of the five years | 
of complete observations the number of hours were 680, 576, 898, 970, | 
and 634—ihus also showing enormous differences among the separate | 
ears. 
: As regards diurnal phenomena, the hourly variation for each month | 
has been calculated for temperature, pressure, humidity, cloud, rainfall, | 
wind-velocity, and sunshine. Results of great value have been arrived © 
at, for which, however, we must, in this brief report, refer to the volume — 
herewith submitted to the Association. ' 
In addition to the usual routine work of a first-order meteorological | 
observatory, other observations have been carried on, mostly of a novel | 
character, tor which the observatory affords exceptional facilities. 1 
The rapid formation of snow crystals, in certain states of weather, | 
from fog, on the observatory and every object exposed to these drifting | 
fogs, has been carefully observed and investigated by Mr. Omond, With 
these rapid accretions, the cups of Robinson’s anemometer are no longer 
hemispheres, but irregular hollow bodies, bristling all over with pointed 
crystals, and the arms increased to many times their original thickness, 
and thus the whole instrument soon becomes a mass of immovable snow, 
and further observation is rendered impossible. The thermometer box, 
with its louvre boards, similarly becomes serrated with rows of teeth, 
which quickly coalesce into a solid, and the instruments are no longer in | 
contact with the free atmosphere. In these circumstances a fresh box 
is put out. It is thus that at observatories such as Ben Nevis, owing to 
these accretions of ice on the thermometers, the continuous or hourly 
registrations of the temperature of the air must be for ever impossible. | 
In truth, such observations must always be eye observations, where the 
observer personally sees that, previously to the recording of each observas — 
tion, the thermometer is in contact with the free atmosphere, and is not 
sheltered from it by a coating of ice. The importance of thermometri¢ 
observations is emphasised by the circumstance that without them the 
barometric observations are of comparatively small value. Ben Nevis is 
the only observatory that has hitherto coped, and that successfully, with 
this all-important department of the work of a high-level observatory 
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