52 
On the Errors to which the 
[Feb. 
this, however, is not now so common as it was some years ago. More attention has 
of late, been paid to this part of the subject, by instrument makers. The correctness 
of division is easily ascertained, and generally, in most instruments now in use, it is 
sufficiently exact. But supposing the scale perfect, and the tube properly filled, a 
fertile source of error is the different specific gravity of the mercury employed. It is 
much to be desired, that philosophers would agree to use mercury of some definite 
density j for until this is the case, no consistency among different instruments is to 
be expected. I would suggest 13,6 as a very convenient density for general use. 
Mercury of this sort is procurable without much trouble, and it does not soil the 
tube. In the instruments now employed, it may be found of all sorts, from 13,2 to 
13,6, and occasionally 13,65. It seldom, I believe, in ordinary instruments, exceeds 
the last number. It might, no doubt, by careful purification, be obtained of greater 
density, but would be more difficult to be procured ; and though it might be used in 
the more expensive instruments, yet it would not be used in the common ones, 
which would thus not be so directly comparable with the standard ones. The 
particular density is of little importance compared with uniformity, and 13,6 is 
a number more easily remembered than one having a greater number of figures. 
A column of 30 inches of mercury of 13,6 will exactly balance a column of 34 
feet of water. The convenience of expressing correct results in whole numbers is 
not to be overlooked. 
The next source of discrepancy among different barometers, is difference in the 
diameters of the tubes. But as the amount of this is constant in the same instru- 
ment, and as the value of it is known for all diameters, it is unnecessary to say 
more about it. Another very common source of error, is the imperfection of the 
vacuum. The vacuum may be imperfect from three causes, 1st, from atmospheric 
air ; 2d, from moisture ; 3d, from the vapor of the mercury itself. 
If the error arise from contained atmospheric air, the amount of it may be deter- 
mined in all cases without much trouble. If it arise from contained moisture, the 
amount of the error may likewise be calculated in all cases ; but the calculation 
would be very troublesome, as it Avould be necessary, in the first place, from compa- 
risons at different temperatures, with a correct barometer, to ascertain the quantity 
of moisture m the tube, and then from the pressure of vapor at the temperature, and 
the capacity of the tube void of mercury, to find what pressure it would actually 
exert, and hence we should have the consequent depression of the mercury in the 
tube. A formula for this purpose might be easily given; but the best of all modes 
of correcting this error, is to expel the moisture, by refilling and reboiling the mer- 
cury in the tube. The depression arising from the vapor of the mercury, may ap- 
pear at first to be inconsiderable. In Barometers which have stood several months 
in the same place, I have found it amount to .020 and upwards. As far as my ob- 
servation extends, the error from this cause is nearly insensible for the first week or 
ten days ; and at the end of about a month, it may not exceed .003 or 005, but it 
increases according to the time that the instrument is allowed to remain un- 
moved I have no means of determining the ultimate amount which it would attain; 
toni? y “ t Pe “ dS ° n , the temperature as on the time. The remedy is very 
the vaVor Tn y r e f e i merCUry *° “ cend to “*0 top of the tube, and 
wwThTo" t Wh ° Uy COn f e " 8e 1 r™ ld recommend, therefore, that in all cases 
wnere a barometer is in constant use, the merenrv a * , , 
the tube nn™ Q „ , „ , * , . CUI 7 be caused to ascend to the top of 
ruDe, once a week at least, and thus the ,,rr.„ij u , , „ . , 
came of much importance. I have examined severd barl ? “"7 ^ ‘ 1 
have invariably found them to stand hirter bv acn 7 m reSpeCt ’ T 
mercury had been made to fill the tube thin thev TT ** 
j nan they did previously ; and I have no 
