252 Prof. J. Lovering on the Aneroid Barometer. 
the barometer had been subjected to excessive heat or cold and 
was then restored to a medium temperature, manifests the impor- 
tance of having the thermometer inclosed as the rest of the instru- 
ment. ‘The standard of temperature adopted was 55° Fah. to ac- 
commodate the scale of the mercurial barometer. 
‘he result of this series of comparisons is contained in Table 
II. Although the agreement is much closer than with the low 
ranges, it falls far below the requirements of nice scientific inves- 
tigations. Mr. David Purdie Thompson in his very recent “ In- 
troduction to Meteorology,” has the following paragraph. ‘‘ Upon 
comparison of indications made with the aneroid barometer—not 
corrected for the particular temperatfire—and a very perfect mer- 
curial barometer, given by Mr. Dent, we find that from forty-nine 
observations made between the 6th of January and 23d of Feb- 
ruary, 1848, the mean difference was 0-037 of an inch, the aneroid 
being in excess; and from sixty similar observations made wit 
a standard barometer, during December, 1848, and between the 
3d and 31st of January, 1849, the mean difference amounted to 
0-026 of an inch, the mercurial being, in this case, in excess over 
the aneroid barometer. Combining these observations (109 in 
number) a mean difference amounting to 0-0025 of an inch is 
found to exist, the indications of the aneroid being in excess. 
For general use the instrument is thus shown to be well suited; 
for the measurement of heights it is peculiarly adapted, from its 
portability and comparative strength; and for nautical purposes 
we know of no better instrument.”—p. 448 
Now it will be observed that the mean difference in the twenty- 
eight comparisons of the two barometers which I have given 
amounts to only -040 of an inch. So far as can be inferred 
from the value of the mean differences, the comparisons were as 
satisfactory as in the first set given by Mr. Thompson. Still the 
single differences are large; whether larger or smaller than in 
Mr. Dent’s observations Iam not able to say, as Mr. Thompson 
has not given the individual differences. Provision has been 
made in the construction of the instrament for diminishing the 
“mean difference as we alter the general rate of a chronometer. 
If the mean difference is eliminated from the comparisons an 
the remaining differences are placed in a column as in ‘Table I, 
they manifest by the signs of plus and minus the irregularities of 
the instrument in small ranges and the errors to be expected from 
these irregularities in single observations. I have arranged the 
same observations in Table III, according to the sign and the 
value of these remaining differences. From the sign of the dif- 
ferences it appears that when the barometers fall the aneroid falls 
most, and when the barometers rise the aneroid rises most. 
other words, the aneroid index moving on either side of the point 
where it agrees with the mercurial barometer moves too fi 
