192 



PROFESSOR FORBES ON THE TEMPERATURE OF THE EARTH. 



1816 to 1821, which I find preserved in the Natural Philosophy Collection; but 

 my inquiries led me to discover that most of these cm'ves had already appeared 

 in the 4th volume of Constable's Edinburgh Magazine for 1819, where they have 

 remained apparently quite unknown to scientific men, together with the details 

 of the observations on which they were foimded, from May 1815 to March 1819, 

 and an explanatory article, which I understand was written by Mr George Bu- 

 chanan, civil-engineer. Tliese observations, particularly at the commencement, 

 were not made with great regularity, sometimes only a single observation being 

 made in a month, at other times as many as eight. Indeed, they have not ap- 

 peared to me to be worth reprinting at length ; but I have condensed into the fol- 

 lowing Table the information which they contain, shewing the mean monthly tem- 

 perature, at different depths, for nearly four complete years. 



Table II. Mean Results op Leslie's Observations. 



III. Construction of the Instruments, 



In commencing the observations at Edinburgh, it was determined that the 

 thermometers should consist of three sets of four each, the lengths increasing in 

 geometrical progression, and the localities being fixed so as to embrace within a 

 small radius, as great a variety of soil as possible. As the deepest thermometers 

 were to be sunk to a depth of 24 French feet (25-6 English), and the portion of the 

 tube, including a column sufficiently long to register the variations of temperature 

 of the fluid Avhich filled the ball (alcohol), must project a foot or two above the 

 surface, the construction, graduation, and depositing of such unwieldy instru- 

 ments, were attended with no small practical difficulty. The execution of the 



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