332 
MR. AIRY’S ACCOUNT OF THE BARTON EXPERIMENT 
and the transmission of the detailed observations to me was in consequence delayed. 
An inequality of temperature, which began accidentally, was thus allowed to exist 
too long to admit of correction. On comparing the mean of all the upper tempe- 
ratures with the mean of all below, I find that the lower station was warmer than 
the upper by 7°' 13 Fahr. (the upper and lower means being 55°‘75 and 62°'88). 
Therefore, if the adopted coefficient of thermal correction is erroneous, the result of 
these experiments will be erroneous by the amount of that error on a range of tem- 
perature of 7°‘13. There may yet be opportunity of verifying the coefficient of tem- 
perature-correction. 
Section VII. — Measure of the difference of level of the two P endulum- Stations ; sur- 
vey of the neighbouring country ; and cornputation of the difference of attraction 
of the Eartlis shell on the two pendulums, upon an assumed value of density. 
49, For measuring the depth of the mine, Mr. Simms furnished an iron wire 100 
feet long, with an attached scale at each end. After the operation, this wire was 
returned to Mr. Simms, and was found to have preserved its length sensibly unal- 
tered. It was used by Mr. Arkley to measure the depth, in the following manner. 
Mr. Arkley placed himself on the top of the “Cage” in which the coal-trams ai’e 
conveyed from the bottom of the shaft to the top, and, when it was lowered 100 feet, 
an assistant attached himself to the strong wire-rope on which the Cage is suspended. 
The assistant carried the upper end of the measuring- wire, and in the first instance 
held it level with the top of the shaft (the steam-engine used for raising or lowering 
the Cage being stopped) ; Mr. Arkley noted the position of the lower end of the 
wire (to which a small stretching weight was attached), and fixed a small nail in 
the brattice opposite to that lower end. Then the Cage was lowered 100 feet; the 
assistant held the top of the wire to the nail so fixed, and Mr. Arkley drove a second 
nail in the brattice, opposite the lower end; and so on to the bottom ; the last frac- 
tion of 100 feet being measured with a measuring tape. After this, the levels of the 
pendulum-stations were referred to the top and bottom of the shaft by the ordinary 
surveyors’ levelling operations. 
The following is an abstract of the result ; — 
ft. in. 
Depth of the shaft 1263 6 
Rise of the floor of the mine from the point of measurement at the bottom 
of the shaft to the floor of the lower pendulum-station . . . . . 
Fall of the surface from the point of measurement at the top of the shaft 
to the floor of the upper pendulum-station ......... 
Distance between the floors of the upper and lower pendulum-stations . 
03 
2 7i 
1256 1 
50. For computing the attractions which determine the difference of gravity at the 
