194 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 
instruments were suspended during winter in a staircase, with their bulbs in 
water, and the temperature of the air surrounding the stems was carefully noted, 
and a correction applied for any difference between it and that of the water. The 
staircase was artificially heated through a few degrees, and after being left for a 
night, a second point was fixed. The temperatures of the water were determined 
by the mean of three standard thermometers, which agreed extremely closely 
indeed, when the error of their freezing points was corrected. The first was a 
standard by TroucHTon and Simms, belonging to and corrected by myself.* The 
second was a standard constructed by Mr Apis for the Royal Society of Edin- 
burgh; and the third a standard having very long degrees, constructed by Mr 
Aptis for his own use. The first pair of observations were made by Mr ADIE 
senor alone, and scratches marked on the tubes at temperatures corresponding 
to 41°73 and 50°77 by the mean of the standard thermometers. To verify these 
results I made two additional comparisons with the assistance of Mr ALEXANDER 
ADIE junior, at temperatures 41°-97 and 46°42, which agreed by interpolation 
remarkably closely with those of the first experiment, considering the difficulties 
of the observation. In only one case (the 13 feet thermometer for the Experi- 
mental Garden), was the difference at all considerable. A mean result was 
adopted. The length of 1° in the 24 and 12 feet thermometers being from 1 to 
2 inches, and divided into 20ths, 13s can be easily read by estimation. In the 
others the approximation is less.+ 
IV. Localities—Sinking of Thermometers. 
Whilst the preparation of the thermometers was going forward, I had holes 
prepared for inserting them in the positions already fixed on with reference to 
the geological peculiarities of the soil. These were— ° 
1. In the Observatory enclosure on the Calton Hill, at a height of 350 feet 
above the sea. The rock is a porphyritic trap, with a somewhat earthy basis, 
dull and tough fracture. The exact position is a few yards east of the little 
transit-house. There are also other buildings in the neighbourhood. The ground 
rises slightly to the east, and falls abruptly to the west at a distance of about 
15 yards. The immediate surface is flat, partly covered with grass, partly with 
oravel. 
* Philosophical Transactions, 1836, p. 577. 
+ More lately Mr Anpiz has constructed two sets of thermometers resembling these, one, extend- 
ing to 24 French feet for Greenwich Observatory ; the other, including only the 12 feet thermometer, 
for Mr Catpecort of Trevandrum in India. Both of these sets of instruments were fortunately trans- 
ported to their destinations without any accident. The graduation of the Greenwich instruments was 
performed by myself, and a much larger number of points fixed than above described. The result was 
examined, and the scale determined, by a simple method of graphical projection and interpolation, 
which led to the most satisfactory results; I should, therefore, recommend this method to others under- 
taking the same tedious and difficult operation. 

