J. H. Alexander on the Tension of Vapor of Water. 223 
It will be observed that I have left out of this comparison, the 
last observation of the Academy ; because it was the very utmost 
point which the apparatus could carry, and because it might there- 
fore be expected to be affected by the untrustworthiness which 
forbade the series from being extended farther. I have also neg- 
lected the last observation of Arzberger; which, compared with 
the Academy’s, is in error more than n 10°,—a deviation sufficient 
at all; because, if we admit the series just now tabulated, his re- 
sults are altogether too high. He may, however, be compare 
with himself, in the two results .he has. recorded for his last ob- 
servation. ese two different pressures accuse a corre ing 
difference of temperature of 0° 63 F. ; a possible error, not so 
materially less than what we have found as the maximum that 
can attach to the formula. The Franklin Institute experiments, 
which correspond closely with Ure’s, I have omitted for a similar 
reason ; they do not profess, even, to read nearer than 0°°25F. 
fom may, however, for illustration be. eo: mapared. with those of 
Academy, as under : ‘ 
7 pressure, 145-15 inches ; temp. —* 305°- - — calculated, ‘ai 
Institu 154-28. 305 -50 . 
Differences,” #8 MESES MED ‘ oe ‘ "40-29 
Discounting the observed difference from the eelanlotat one, we 
have left 4°-11F., as the error of one or the other series; an 
amount nearly four times that of the. * foctonth 
It is manifest that the comparative error of the formula is only | 
approximate ; because it is based, im each case, upon only one ob- 
servation instead of upon the gpenbines mean of all the observa- 
Bane or, rather, the mean of the difflerences.at every. epoch ob- 
ved. Also, it can only be called an error, upon the assump 
ton of the mean of all the experiments resulting in absolute ac- 
racy ; am assumption by no means to be made ; for, in general, 
he utmost that can be done for any experimental asking is to de- 
termine the limits of its neeessary or accidental e Such a 7 
research and determination, I have thought the pecncgt formula _ 
of sufficient interest to warrant, ‘The account, which is it “" 
the promised and proper conclusion of the present paper, walla ; 
pear in a fetes Fn rag this Journal. 
