TRANSACTIONS Of THE SECTIONS. 
581 
requires address. 3. The air-speck will now be evident : if 
its length, when made to enter the tube, is less than half a de¬ 
gree, the instrument may be considered as fit for immediate 
use, as soon as, by a process of refrigeration, the right position 
of the air-speck is obtained. 4. If the length of the air-bubble 
exceed a degree or two, it may be easily diminished, by, first, 
causing the separated portion of the column to pass into the 
empty bulb ; and, secondly, returning it into the tube, after 
exposing the mercury in the bulb to some augmentation of 
temperature. 
Finally, by this process a dexterous workman may easily con¬ 
vert any good common thermometer, of a fine bore, into a self- 
registering instrument. 
James D. Forbes, F.R.S.E. &c., announced that he has a 
Register of Observations of the Thermometer nearly ready to be 
communicated, which has been kept, for a period of three years, 
in Scotland, at an elevation of 1100 feet above the level of the 
sea. 
Mr. Forbes also exhibited and explained the construction of 
an improved portable Barometer , which had been recently ex¬ 
ecuted for him by Mr. Robinson of Devonshire-street, and in 
which he had sought to combine the requisites of an exact zero 
for the scale, an accurate indication of the temperature of the 
mercury, steadiness during observation, and convenience and 
security in travelling. 
\The Report of the Experiments which were made at York by 
the Secretaries of the Yorkshire Philosophical Society, on the 
quantity of Rain falling at different Heights, having been acci¬ 
dentally mislaid, will appear in the next Volume of the Reports.] 
7. GEOGRx\PHY.—GEOLOGY. 
On the Compilation of a General Table of Altitudes of Places 
in Great Britain and Ireland. By Ben j. Bevan, F.R.S. &c. 
It is well known, that in the common topographical publica¬ 
tions, two only of the co-ordinates, out of the three requisite 
for defining the position of a spot on the surface of a district, 
have yet been given; that is, the latitude and longitude ; 
whereas, to define the spot completely, the altitude is required. 
To have attempted to supply this defect many years ago 
