measurement of Snowdon by the Thermometric al Barometer. 303 
General Roy's measurement trigonometrically makes it 
2371 feet; barometrically 2391,8 feet. The three measure- 
ments in this instance, do not agree so well as at Snowdon : 
whether this arises in any degree from the different forms 
of the two mountains, rendering the point of observation less 
definite in the one case, I will not pretend to say ; Snowdon 
terminates in a point, Moel Elio has a large bare summit. 
To save the trouble of reference to General Roy's Paper, 
Philosophical Transactions, 1777, p. 771 , 1 give here from that 
Paper, a part of the Table for correction on account of the ex- 
pansion of the column of air between two stations at different 
temperatures, in thousandth parts of an observed height. 
Table III. 
28,5 
29 
29 
>5 
3 C 
5 
30,5 I 
difference. 
difference. 
difference. 
difference. 
difference. | 
12° 
44,7 
45,6 
46,6 
47.5 
48,4 
2,19 
2,24 
2,28 
2,33 
2,37 
22 
22,8 
23.3 
23.7 
24,2 
24,7 
— 
2,28 
2.33 
04 
00 
2,42 
2,47 
32 
Subtract above. 
Add below this line. 
-f 
2.37 
2,42 
2,47 
2,52 
2,57 
42 
2 3>7 
24,2 
24,7 
25,2 
25,7 
2,46 
2,51 
2,56 
2,61 
2,66 
5 2 
4 8 >3 
49.3 
5°>3 
5>>3 
52,3 
2,55 
2,60 
2 ,66 
2,71 
2,76 
62 
00 
75.4 
76,9 
7 8 .4 
79*9 
2,51 
2,56 
2,61 
2,66 
2,71 
72 
98,9 
100,9 
103,0 
105,0 
107,1 
2,46 
2,51 
2,56 
2,61 
2,66 
82 
123.5 
126‘I 
128,6 
131,2 
133,7 
The instrument with which these experiments were made, 
had been improved from the original construction as de- 
scribed in the Philosophical Transactions, and I shall men- 
tion the particulars of difference. The thermometer itself is 
straight, and carried up the middle of the scale. The index 
moves by hand on a square rod on one side of the glass tube, 
