90 
Proceedings of Royal Society of Edinburgh. 
winds of all velocities up to at least 150 miles an hour, whereas the 
other is in a building where either calms or light winds only at the 
time prevail — so that this barometer may be regarded as recording 
the pressure of the free atmosphere. It was therefore resolved to 
institute a comparison between the sea-level pressures of these two 
barometers, employing only those cases when winds at the Fort- 
William Observatory were light. 
The scale used on Ben Nevis for the observations of the force of 
the wind is a modification of Beaufort’s scale, 0 to 12. Much 
attention has been given to ascertain the wind’s rate in miles per 
hour, corresponding to each of the figures of Beaufort’s scale. For 
this purpose, a modification of Robinson’s anemometer was de- 
signed by Professor Chrystal for the Observatory ; and during the 
times the instrument is not frozen up in a thick covering of ice, the 
comparisons have been made. These have been discussed by Mr 
Omond in a paper read to the Society. The comparison is given 
at the top of Table I. 
The reductions of the barometric readings on the top of Ben 
Nevis to sea-level have been made by Table VIII. prepared for the 
purpose, as given in the volume of the Transactions recently pub- 
lished,* and the readings at Fort-William in the usual way. The 
differences of the two reduced readings were then entered in columns 
headed 0, 1, 2, 3, &c., and according to the wind force at the Ben 
Nevis Observatory at the time. Table I. gives the mean differences 
for each wind force for each wind ; and the figures in the second 
half of the Table show the number of observations from which each 
mean difference has been calculated. The comparison for the six 
months was made from the hourly observations at both Observatories, 
from August 1890 to January 1891. But since this period gave too 
few observations for the higher wind velocities for good averages, 
the observations from January 1885 to July 1890 were utilised for 
the five hours of the day at which corresponding observations were 
made at Fort-William. Only the wind forces from 5 to 11 have 
been thus utilised, and the results have been incorporated with 
those for the six months, and entered at the foot of Table I. In all 
4596 of the Ben Nevis observations have been reduced to sea-level 
for these comparisons. 
* Trans. Roy. Soc. Edin vol. xxxiv. pp. 60-61. 
