[ 325 ] 
than Mr, B.rd’s Icale ; on which account the length of the degree fir on Id be further 
duninifhed by — P art 5 or 3 feet, which added to 10 feet, the correction required 
on account of the difference of Mr. Bird’s fcale and the Royal Society’s ftandard, gives 
13 feet to be Cub trailed from the length of the degree calculated above. The whole 
correction will perhaps be thought fcarce deforcing of notice, efpecially as an error of 
■only 1" in the celeftial meafure would produce an error of nolefs than 67 feet in the 
length of the degree. Moreover it is probable that the length of a degree has been 
■already taken IO or 20 feet too fhort, by placing the point C too far to the Southward ; 
which would about balance the fmall correction in queftion. Therefore, all things 
being confidered, the length of the degree may be Rated as given above, viz. =2:363763 
Englifh feet or 36904! Paris toifes. It mult, however, be obferved, that the accuracy 
of this reduction into Paris toifes depends upon a fuppofition that the length of the 
French toile, which is of iron, was laid off by the gentlemen of the Royal Academy of 
Sciences, upon the brafs rod feat over to them for that purpofe by Mr. Graham (which 
was afterwards returned to him) ; in a room where the heat of the air anfwered to 62 of 
Fahrenheit’s thermometer, or 13 of Reaumur’s, or nearly fo, which is probable 
enough, hut is a point that does not appear to have been ascertained. For, on account 
of the difference of expanfion of brafs and iron ; 2 rods made of thofe metals, however 
accurately they may be made of equal lengths at firft, will only agree together after- 
wards in the lame temperature of the air in which they were originally adjufted toge- 
ther. It is fortunate that the uncertainty in the prelent cafe is but fmall, fince 20 ° 
difference of Fahrenheit’s thermometer or io° of Reaumur’s produces, according to 
Mr. Smeaton’s experiments, a difference of the expanfions of brafs and iron of only 
TTT-cro-th part, which would caufe an error .of only 27 Englifh feet or about 4 Paris 
toifes in the length of the degree. 
It is however to be wifhed, that the proportion of lengths of the French and Englifh 
meafures might be again ascertained by another careful experiment, in which thC 
temperature of the air, as Ihewn by the thermometer, might be noted at the time. 
[See the Map of the Country, where the foregoing Obferyations were made, T a b . XIV J 
POSTSCRIPT, BY THE ASTRONOMER ROYAL. 
H A V IN G, fome time ago, acquainted M. De la Lande, of the Royal Academy 
of Sciences at Paris, by letter, of this meafure of a degree of latitude in North 
America, and at the fame time expreffed my doubts about the certainty of reducing it 
to French meafure, from the proportion of the Englifh to the French foot found by 
Mr. Graham ; principally becaufe no notice had been taken of the height of the ther.- 
mometer at Paris, when the length of the French iron toife was laid off upon the brafs 
rod fent thither by Mr. Graham, whence the proportion of the two meafures was af- 
terwards determined by him •, and having alfo mentioned my opinion of the expediency 
of making another experiment of the proportion of the two meafures, in which every 
neceflary circumftance fhould be noted ; and that I might probably requeft the favour of M. 
De la Lande to take the trouble to caufe a French toife to be made for me, and to fee it ex-> 
aCtly adjufted to their ftandard, and then fent to me ; he has beenpleafed to lend me two 
toifes, 
