24 Proceedings of Royal Society of Edinburgh. [dec. 6, 
which, when the terminal conditions are assigned, gives the steady 
distribution of temperature. The motion of the gas is analogous to 
that of liquid mud when a scavenger tries to sweep it into a heap. 
The broom produces a general translation which is counteracted by 
the gravitation due to the slope, just as the translation of the gas is 
balanced by the greater number of particles escaping from the colder 
and denser layers than from the warmer and less dense. 
In thermal foot-minute-centigrade measure, the conductivity of 
air, at one atmosphere and ordinary temperatures, appears from the 
above expressions to be about 
Jin?' ! 
or about 1/28,000 of that of iron. Ho account, of course, is taken 
of rotation or vibration of individual particles. 
4. Fog Bow observed on Ben Nevis, 22nd October 1886. 
By K. T. Omond, Supt. B.N.O. 
(See Proceedings for June 20, below.) 
5. Temperatures at Different Heights above Ground at Ben 
Nevis Observatory. By R T. Omond, Supt. B.N.O. 
During part of the recent summer (1886), in addition to the 
ordinary temperature observations here, a set of readings were 
taken in a Stevenson screen, at a height of 112 inches above 
ground — that is, at about two and a half times the standard eleva- 
tion of 48 inches. The high level screen was mounted at the top 
of a stand used to carry the thermometers in winter, and consisting 
of two stout upright posts or battens, with cross bars between them 
at every 2 feet or so ; the screen is placed with the lower edge of 
the back resting on one bar, and is tied to the one above it or to 
the side posts. The screen used on this stand is smaller than the 
standard Stevenson screen, but it is constructed in exactly the same 
manner, with double louvred sides and the bottom open. It 
measures inside 10 inches broad by 6 inches deep and 15 inches 
high. The low level screen, which is mounted on four legs in the 
