HEAT OF EVAPORATION OF WATER. 329 
rately determined, but observational errors of this kind are precisely those where a 
close approximation is secured by taking the mean of a group. If the middle observa- 
tion of each group is compared with the mean, it is evident that an individual 
observation might be in error by as much as 20 seconds (the greatest difference in the 
above table is 12 seconds), but it is improbable that the mean is in error by more than 
5 seconds. The example given is the one which had the slowest rate of rise, and hence 
the discrepancies are more marked than would otherwise be the case. As the total 
time of the experiment was about 2 hours 20 minutes, an error of even 10 seconds in 
the mean of each group would affect the result by but 1 in 800, and as an error of 
1 in 50 in the value of Q, would only affect my final values of L by 1 in 5000, the 
above order of accuracy was more than sufficient. 
I will now give the reduction of the observation in the above table. 
REpvctIon of Stirring Experiment No. I. 












: dl a brs Correction 
Bridgeawire |e Time jleneo =| Heeuleine)| ge, |\Coumection 00) tor eo 
yaoi Ue meter II.| inrange.|| @% bridge-wire. temperature || orrected. 
‘S) 2 bridge-wire. 
590°6-595'0 | 2154 +-02 Stole 002094. --0°4 +1°7 002090 A 
595°0-599'5 | 2528 — 014 — 07 001753 —12 +15 001753 || B 
999°5-603°7 | 2772 —-030 — 17 | 001454 +43 +12 001460 || C 



Since A, B, and C should, if the exterior temperature (No. II.) had not changed, 
fall on a straight line, the most probable path is obtained by taking $ (2A + B) and 
3(B + 2C) as the rate of rise at the corresponding bridge-wire readings, hence 
we get 


Bridge-wire. d0, dt. 
59428 001978 
600°15 001558 

Now the bridge-wire null-point = 598°35 + ‘036, (see p. 288). 
point at 40° = 599°55. 
We get ‘001600. 
Hence 
Therefore null- 
We can now deduce the value of d0,/dé at this null-point. 
(d0,/dt), = 001600, 
when 7, = 5°299 and @, is measured in millims. of the bridge-wire scale. 
I think that there is no necessity to give details of the remaining experiments ; 
MDCCCXCV.— A. 2U 
