196 PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



Strong wooden boxes painted green, having doors open to the north side, were 

 then fii-mly fixed over the projecting stems, which boxes were afterwards pierced 

 with holes, in order to secure a free ventilation. Small thermometers graduated 

 to whole degrees were hung in air within the boxes ; and afterwards (May 1838) 

 other thermometers were placed with their bulbs just covered by the soil within the 

 boxes. These last thermometers are referred to in the tables as t^. After the in- 

 struments were finally placed, slight metal scales were attached to the respective 

 tubes with fine copper wire. I should add that, for the defence of the bulbs and 

 the capillary tubes, at their inferior extremities, before mentioned, they were half 

 inserted into tin cylindrical boxes filled up with plaster of Paris. 



I had hoped to have commenced the observations with 1st January 1837 ; 

 the unexpected difficulty experienced in boring the holes, and subsequently the 

 severe weather, prevented the insertion of the thermometers before the 18th Jan- 

 uary in the Experimental Garden, the 20th on the Calton Hill, and the 21st at 

 Craigleith Quarry ; the whole was happily accomplished without the slightest ac- 

 cident. In all my aiTangements, I was aided by the civility of the Directors of 

 the Astronomical Institution and of the Experimental Gai'den, by the astronomer 

 Mr Henderson, and other ofiicial persons. 



V. Observations and Observers. 



Not the least of the diflBculties of carrying on such observations as the pre- 

 sent has ever been found to be, that of getting perfectly trustworthy and zealous 

 observers. In this matter I esteem myself particularly fortunate. Professor 

 Henderson undertook, in the kindest manner, the personal superintendence of 

 the thermometer placed on the Calton Hill in the Observatory grounds. At the 

 Experimental Garden I received the services of Mr James M'Nab, the superin- 

 tendent ; and at Craigleith, those of Mr Mackintosh, whose official connection 

 with the quarry ensures his constant residence. The observations were made 

 weekly, and were registered in degrees and hundredths of Fahrenheit's scale (by 

 approximation). The general superintendence which I have been able to give, 

 assured me that aU the observations were made, not only with fidelity, but with- 

 out any sensible eiTor arising from want of familiarity of two of the observers 

 with instruments so minutely divided. And, were other proofs wanting, fortu- 

 nately the ultimate projection and comparison of the three sets of observations 

 affords the most perfect check upon any considerable inaccuracy, either in the 

 obsen'ations or computations. 



With a view to check any permanent change in the reading of the instru- 

 ments, such as might arise from a permanent displacement of the freezing point, 

 I had a spirit thermometer constructed with a bulb similar to those buried, with 



