C S 9S 1 
XVIII. On the measurement of Snowdon, by the Thermometrical 
Barometer. By the Rev. F. J. H. Wollaston, B.D. F.R.S. 
Read June 29, 1820. 
The Royal Society did me the honour to notice in their 
Transactions, 1817, p. 183, the description of a Thermo- 
metrical Barometer, by which it was conceived that acces- 
sible heights might with convenience be measured ; and may 
therefore not be uninterested by the account of an actual 
measurement made with it. Having occasion last summer 
to visit Carnarvon, which would afford an opportunity of 
trying the instrument on the known height of Snowdon, and 
being aware that in 3550 feet the variations of the boiling 
temperature were not to be considered uniform, as they 
might in small elevations, on which alone I had before tried 
the experiment, I wished to provide myself previously with 
a table for making the necessary corrections ; and from Dr. 
Ure's paper, Philosophical Transactions, 1818, p. 338, was 
supplied with data for the calculation. The law, which he 
discovers by approximation and lays down, is this : that, the 
elastic force of the air, or length of the column of mercury 
in the barometer being supposed 30 inches when water boils 
at 212 0 of Fahrenheit, will be the length of the column 
at 202°; i 23 3 ° t the length at 192 0 ; and so on progres- 
sively, adding another increased divisor for every io° of ther- 
mometrical temperature. 
For my purpose it was necessary to calculate the varia- 
tions at smaller intervals than ten degrees, and particularly 
to obtain those between 212 0 and 202°, by getting a series of 
