202 Gen. Roy’s Account of a 
have determined the length MP, in the map, being the fpace 
comprehended between the parallels of M and Greenwich. 
In the fecond column are arranged the computed dimendons 
appertaining to the earth as a fphere, fuppofing its femi-diameter 
to be a mean between the longed; and fhortedof M. Bouguer’s 
fecond fpheroid. It is from the magnitude of this fphere that 
I compute the degrees of a great circle for the fides of l'pherical 
triangles. By adverting to the errors or differences between the 
meafurement and computation, in their refpeclive places, it 
will obvioufly appear that the earth differs very confiderably 
from a fphere : for although the arc M Perpignan of 8°| only 
exceeds the truth by 609 fathoms ; yet an arc of equal length 
at the equator, viz. 8°. 33 x 374.6, would give an excefs of 
31 20 fathoms ; and at the polar circle 8°33 x 335.2 would give 
a defedl of 2792 fathoms. 
After the Inhere follow feven ellipfoids of different degrees 
of oblatenefs, from the fird, whofe femi-diameters have to 
each other the ratio of 179.047 to 178.047, to the feventh, 
where it is only that of 540 to 539. On the principles which 
have ferved as the foundation of the fird and fecond, it will be 
neceffary to make fome remarks ; but as to the others, a few 
words will fuffice for each. 
With regard to the fird: ellipfoid, fuppofing the earth to be< 
homogeneous, it is well known, that the ratio of its femi- 
diameters may be found, by comparing with each other the 
lengths of the pendulums that vibrate feconds in different lati- 
tudes ; which lengths are deduced from the feconds of accele- 
ration, that the pendulum, fo adjuded, and unalterably fixed 
as to length, at the equator, would perform in 24 hours, on 
being fuccedively transported to different latitudes, as far as 
the pole, where the force of gravity being the greated, 
the acceleration would like wife be the greated. The calcu- 
3 lations 
