[ 2 37 ] 
attending to the difference between Paris feet and 
London feet ; and the fame method muft be taken 
to get rid of the error in both cafes : vre muft 
afeertain the difference of the fades we have applied, 
and make allowance for it : we muft know the 
length of the one in parts of the other ; that is, in 
the particular cafe in queftion, we muft be able to 
determine, from the obferved height of the baro- 
meter, when the quickfilver is of any given tem- 
perature, what its height would have been, at the 
fame place, and at the fame time, if its tempera- 
ture had been any other, that may be affigned. I 
have thought it neceffary to be thus minute, in ex- 
plaining the principles, upon which the corredlion in 
queftion depends; becaufe it is a point, which is 
likely to be mifunderftood, though in itfelf of no 
great difficulty. M. de luc himfelf hath fallen 
into a miftake, not with refpedl to the quantity of 
the error, but the manner of allowing for it ; which, 
however, is of no other bad confequence, than that 
of lengthening the calculation unnecefftirily. He 
imagines that a particular temperature of the 
quickfilver is neceffary, that the error fhould be 
nothing ; and to this he always reduces the obferved 
heights of the quickfilver, in the barometer, at both 
ftations W. But the refuit of the computation 
would, in all cafes, have been the fame, if they 
had been reduced to any other given temperature; 
and therefore, it is always fufficient, to reduce the 
one to the temperature of the other. This little 
overfight I fhould have touched upon with more 
(£) Recherch. furies Modif. de 1 ’Atm. §. 369 — 374. 
relu&ance. 
