MEASUREMENT OF TEMPERATURE. 
225 
Table P.-iii. 
Time. 
Resistance observations. 
Approximate 
temperature. 
Difference 
pt 3 - pti 
Number of 
observation. 
Date. 
Hour. 
R + z 
(R + z)/( R' + z) 
z 
pt 
t 
1886. 
O 
o 
O 
Aug. 24 
12 NOON to 1 P.M. 
15-55 
•61801 
•0875 
603 
661 
+ 0-39 
(1) 
1 to 1.30 p.m. 
14-83 
•61775 
•0839 
562 
611 
+ 0-45 
(2) 
1.40 to 2.20 p.m. 
12-96 
•61702 
•0740 
455 
484 
+ 0-51 
(3) 
2.20 to 3 p.m. 
10-88 
•61600 
•0635 
335 
34-9 
+ 0-50 
(4) 
6.30 p.m. 
5-51 
•61074 
•0384 
26-1 
26 
+ 0-07 
(5) 
Aug. 25 
11.15 A.M. 
5-398 
•61067 
•0379 
20-0 
20 
0 
(6) 
In steam 
12 noon to 1.30 p.m. 
6-800 
•61306 
•0458 
100-17 
100-170 
0 
(7) 
Aug. 27 
(In ice) 
11.30 a.m. to 1.30 p.m. 
5-0537 
•60986 
■0357 
0 
0 
0 
(8) 
The value of t is taken from the curve t—pt — 1'57 {(^/l 00) 3 — (£/ 100 )}. The 
observations are reduced as follows :— 
Let R be the resistance of the wire pt x , R' that of pt z , r the ratio R/R', and z the 
measured resistance of the connecting wires. 
Then r = (R + z) / (R' + z) { 1 + (r — 1 ) z/ R} (z being small). 
Let r stand for 10 O/(R 1 /R o — 1 ), so that r is a constant exactly analogous to 0 O in the 
case of the air thermometer. 
Then pt x = t x (R/R 0 — 1 ); Vh~ T z (R'/R-'o ~ 1 )• 
And the difference pt z — pt x may be put into a form more convenient for calculation, 
namely. 
Ph ~Ph = ( r 3 “ T i) (R/Ro — 1 ) — t 3 (R/R 0 — RVR'o). . . . (T) 
The small difference (r 3 — iq) may be accurately determined from the observations in 
ice and steam, for we have 
!/ r i l/ r 3 = (Ri/Ro R-'i/R o) /100 ; 
whence 
T 3 Tj = t \ t 2 (Ri/Ro R l/R o) A^O, 
and the small difference (R/R 0 — R/R'q) is accurately given by the formula 
(R/R 0 — R'/R'o) = R/R 0 (r — r 0 ) /r, wher e r is the ratio 11/IT. The only quantity, 
therefore, requiring the fullest accuracy of observation is (r — r 0 ), the change of r. 
Since this is small, approximate values of the others will suffice. 
From the observations ( 1 ) and ( 2 ) of Table P.-n. the deduced value of r 3 — t x is 
G '36; from Observations (7), ( 8 ), Table P.— in., r 3 — t x = 6°'23. These values are 
used in the reduction of Tables P.-ii. and P.-iii. respectively. A rise of zero 
MDCCCLXXXVII.—A. 2 G 
