[ 326 ] 
toifes, which he fays are exa&ly adjufted to the ftandard of the toife ufed by Mefl* De la 
Condamine and Bouguer in the meafureof the degrees of latitude at Peru, in order to their 
being compared with theEnglifh meafure. This comparifon has been made by Mr. Bird, 
With his ulual accuracy, while I was prcfent, and alfo examined the fame, fince iny 
account of the length ct the degree of latitude aforegoing was printed ; and the refult is, 
that the longetl ot the two toifes (for there is a fmall difference between them), and 
which has fince been marked with the letter A, is equal to 76,738 inches by Mr. Bird’s 
brafs fcale of equal parts, and the fhorteft toife, which is marked B, 15 = 76,735 inches 
by the fame fcale ; the height of Fahrenheit’s thermometer in the room being 61 de- 
grees. The mean of the lengths of the two toifes is therefore = 76,7365 inches by 
Mr. Bird’s icale. But Mr. Bird’s fcale is th of an inch upon 3 feet fhorter than 
the Royal Society’s brafs ftandard, and consequently th too fliort for the fame 
upon 76,7365 inches ; therefore _._*!_^.th of an inch muft be fubtradfed from 76,7365; 
which leaves 76,7344 for the length of the Paris toife in meafures of the Royal So- 
ciety’s brafs ftandard, in the temperature of 6i° of Fahrenheit’s thermometer. In the 
temperature of 62° it will be a little fhorter ; or it may be taken = 76,734 inches in 
meafures of the Royal Society’s brafs ftandard. This is _^.+_.th or about * d of an 
inch longer than was determined by Mr. Graham’s experiment. Hence it appears, 
that I was miftaken in fuppoftng, in p. 325, that the uncertainty about the true pro- 
portion of the Englifh and French meafures was but final], fince the error in the for- 
mer determination now appears to have been T _L_th of the whole, or equivalent to 
what might have been produced by a difference of 84° of Fahrenheit’s thermometer. 
Whence it arofe I cannot pretend to fay, neither is it very material to enquire ; but the 
fa£t is plain, and fully juftifies the propriety of repeating the experiment. 
I fhall now ftate the length of the degree, meafufed by Mefiieurs Mafon and Dixon, 
ftrft in Englifh feet, according to the Royal Society’s ftandard, and then reduced to 
the French meafure by the proportion juft eftablifhed. 
From 363763 Englifh feet, the length of the degree found by the 5 feet brafs 
ftandard, fee p. 324, I fubtradt 10 feet for the difference between Mr. Bird’s fcale 
and the Royal Society’s ftandard, and 3 feet for the wearing of the brafs rod; and there 
’remain 363750 feet, according to the Royal Society’s ftandard, for the length of the 
degree. But to this it feems proper to add 21 feet, in order to correct the pofition of 
the point C, determined by the fetftor, which cannot be fo certain as that inferred from 
the azimuth of the line PS. See p. 279. Therefore the true length of the degree, 
according to the Royal Society’s brafs ftandard, in the temperature of 62° of Fahrenheit’s 
thermometer, is 363771 feet, or 68,8960 Englifh ftatute miles. To reduce this to the 
meafure of the Paris toife, by the proportion above eftablifhed, fay as 76,734 is to 72, 
fo is 363771 to 341328 French feet, or 56888 Paris toifes, of the ftandard of that ufed 
in the meafure of the degrees of the meridian at Peru. 
The method made ufe of by Mr. Bird, in finding the length of the toifes by his fcale, 
was as follows, which may ferve as a dire£tion for the like purpofe on any future oc- 
cafion. Two brafs pins were drove into a ftrong deal board 4 inches thick, and longer 
than the toife ; and two brafs cheeks were made very fquare, and the ends brought upon 
the pins. The toife was then put in between the cheeks, one of which was made to 
Hide fo as to be eafily brought into contact with the end of the toife, and the other end 
