.50 6 The Account of a 
the readiest, and indeed almost the only means of comparing 
independent deductions from both operations ; the triangle St. 
Ann's Hill, King’s Arbour, Hampton Poor House excepted. 
The distances from the station at the Hundred Acres 
to St. Ann's Hill and Botley Hill, according to General Roy 
(see the 4th and 9th triangles in his account) are 79211,22, 
and 48726,75 feet; and from the 4th, 5th, and 9th .triangles 
it appears, that the included angle at that station is 169° 25' 
21", 25 ; these give 127424,3 feet for the distance of St. Ann's 
Hill and Botley Hill ; this distance, however, is deduced from 
the base on Hounslow Heath, supposing it to be 27404,7 feet ; 
but its mean length, according to both measurements, being 
27404,2 feet, we shall have 27404,7 : 27404,2 : : 127424,3 : 
127422 feet, for the distance of the stations from that mean 
length of the base. 
According to our observations, the distances of St. Ann's 
Hill and Botley Hill from Leith Hill are 88019,8 and 92632,2 
feet respectively, and the included angle for computation at 
Leith Hill 89° 40' 32" ; hence, from our triangles, the distance 
of the stations will be 127420 feet ; which is 2 feet less than 
that from General Roy's triangles. 
Before we compute the distance from Botley Hill to Fair- 
light Down, it will be necessary to premise, that an error has 
crept into General Roy's reduction of the measured base on 
Romney Marsh (see Phil. Trans. Yol. LXXX.) ; which, how- 
ever, cannot be discovered without consulting his account of 
the measurement of the other base on Hounslow Heath. We 
are informed (page 131, Yol. LXXX.), that when the new 
points on the chain were laid off from the original points on 
the great plank in M^Ramsden's shop, Fahrenheit's ther- 
