Determination of Heights. 191 



The following communications \vere read : — 



1. On the Nature, Locality, and Optical Phenomena of Muscae 



Volitantes. By Sir D. Brewster, K.H. 



2. On the Structure of the Lymphatic Glands. By John 



Goodsir, Esq. 



3. On the Determination of Heights by the Temperature of 



Boiling Water. By Professor Forbes. 



The investigations in this paper were made in order to reduce 

 certain observations on the boiling point of water, made by the 

 author in the Alps, in 1842. 



He considered that it has been too generally assumed that the 

 boiling point corresponds to a barometric pressure which expresses 

 the elasticity of steam taken from the usual tables. He, therefore, 

 attempted to deduce the connection of these data by a direct com- 

 parison of cases, in which both the barometer and boiling point 

 were noticed by himself. He finds this result, that the pressures 

 increase rigorously in a geometrical ratio, whilst the temperature of 

 the boiling point rises uniformly. This law is not new, for Deluc 

 arrived at the same result ; but it appears to have been considered 

 by all late writers as unworthy of adoption, and the scale of the 

 elasticities of vapour by Dalton or Ure has been preferred. Now, 

 these elasticities cannot, it is well known, be accurately represented 

 by a geometrical proportion to the temperature; but Professor 

 Forbes finds that the geometrical ratio represents the barometric 

 heights exactly, whilst the tabular elasticities do not. But, farther, 

 since the common barometric formula shews that the pressure 

 varies geometrically, whilst the height above the sea varies uni- 

 formly, we have the same form of relation between the boiling point 

 and the barometric pressure as between the height above the sea 

 and the barometric pressure, namely, that each is as the logarithm 

 of the other. Hence the boiling point falls exactly in proportion to 

 the height ascended, and at the rate of 549.5 feet for 1* Fahr. 



Monday, 20th March 1843. 

 Sir DAVID BREWSTER, Vice-President, in the Chair. 

 The following communication vv^as read : — 

 Papers on Glaciers. No. 3, On the Structure of Glaciers, and 

 the cause of their motion. By Professor Forbes. 



Monday, 27th March 1843. 

 Dr ABERCROMBIE, Vice-President, in the Chair. 



