226 Froceedings of Eoyal Society of Edinhurgh. [march 5, 
Hence the Mean of the Free Paths per particle in T seconds is 
and is thus seen to be, not a true mean but, the ratio of two 
proper means. It can be put, no doubt, in the form 
which seems to agree with De Morgan’s definition. But it must 
be remembered that C is itself only a mean; and thus that CfG is 
not strictly analogous to n^. 
As soon as we admit a departure from the ordinary sense of a 
term, we pave the way for other departures, some of which may be 
at least plausible though many may be grotesque. It is for this 
reason that I said {Trans. K.S.E., 1886, xxxiii. 75) “those who 
adopt this divergence from the ordinary usage must, I think, 
face the question: — Why not deviate in a different direction, and 
define the mean path as the product of the average speed into the 
average time of describing a free path?” 
5. Note on the Compressibility of Glass at different 
Temperatures. By Prof. Tait. 
6. Exhibition of Photographs. 
The Astronomer-Eoyal for Scotland exhibited Photographs of the 
Moon during the recent Eclipse by Mr W. Peck. 
Mr James Durham, Mr William James Bell of Scatwell, the Bev. 
John Stevenson, minister of Glamis, Mr Henry Brougham Guppy, 
Professor Thomas Hudson Beare, Mr Andrew H. Turnbull, Mr John 
Alfred Jones, and Professor John Eerguson were balloted for, and 
declared duly elected Eellows of the Society. 
PEIVATE BUSINESS. 
