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 coiTcction 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 Teoughton and Simms, belonging to and corrected by myself.* The 

 second was a standard constructed by Mr Adie for the Royal Society of Edin- 

 burgh ; and the third a standard having very long degrees, constructed by Mr 

 Adie for his own use. The first pair of observations were made by Mr Adie 

 senior 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 witli 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, tJo can be easily read by estimation. In the 

 others the approximation is less.f 



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 enclosm-e 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 faUs abruptly to the west at a distance of about 

 15 yards. The immediate sm'face is flat, partly covered with grass, partly with 

 gravel. 



* Philosophical Transactions, 1836, p. 577- 



f More lately Mr Adie 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 Caldecott 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. 



