290 PROFESSOR C. PIAZZI SMYTH ON 



beyond all praise ; particularly in their initiating an arrangement for taking them in 

 duplicate, and discussing the results to a hundredth of a degree Fahrenheit, before 

 leaving the Thermometers on each occasion. The names of the said Assistants being, 



in 1879 and 1880, First Assistant, Alexander Wallace, M.A 

 Second Assistant, Thomas Heath, B.A. 



1881 to 1887, First Assistant, Thomas Heath, B.A. 

 Second Assistant, Henry W. Ride. 



1887 to 1888, First Assistant, Thomas Heath, B.A. 



Acting Second Assistant, James Forgan. 



By a preliminary computation of the more salient observations, and an examination 

 of the instruments themselves in their present condition, I have made out a Table of 

 " Thermometer Constants "; and although it will not enable these new Thermometers to 

 compete with the old ones as they were in Prof. Forbes' time from 1837 to 1845, as 

 instruments of the most delicate Natural Philosophy chamber problems, I have been 

 much pleased to find that step by step they have shown their full sufficiency, to keep 

 up the differential historical record of super-annual cycles of temperature, which was in 

 fact almost the only employment of the old Thermometers from 1845 to 1876, when 

 they were destroyed by a human accident from without. 



The new earth-thermometers were constructed by Messrs Adie in 1878, in a closely 

 parallel manner to the printed descriptions of the older thermometers in 1836, and 

 brought to the Astronomer's house in the spring of 1878, where they all had their bulbs 

 put into one and the same tub of salt water, together with a standard thermometer, 

 and the several scales as approximately graduated by Mr Wedderburn, were read off' 

 by myself at different heights up the stairs of the house. A mean of ten sets of read- 

 ings gave for 



« 1 = 48°'41, £ 2 = 48°-31, f = A8°'46 and t*= 48°-39. 



This was so far a testimony, that when the bulbs were all on the same level, and in 

 the same place, they read, with approximation, identically. The stair-case of the house, 

 however, being too warm to enable the ultimate earth temperatures to be experienced 

 there, Mr Wedderburn afterwards erected them outside the house, and perfected the 

 scales there by long and laborious steps of which I have no notes, and can only judge of 

 by subsequent comparative readings. 



Thus the old thermometers were remarkable for every deeper one giving a higher 

 temperature ; and so, on taking the means of the new ones for several years, after they 

 had been sunk into the ground their intended depths — 



new t 1 was found = 46°"29; £ 2 = 45°-93, t 3 = 45°-83; and « 4 = 45°77, 



or each thermometer greater than the shallowest by 



t 1 = 0°-52, t 2 = 0°-l6, £ 3 = 0°-06, and t 4 = 0, 



