437 
Trigonometrical Survey. 
feet. inches. 
A = 100 -f 0,11425 
B = 100 -f- 0,05825 
The comparison of the chains with each other, as related in 
art. hi. together with this determination of their lengths, 
furnish the data necessary for the reduction of the base on 
Hounslow Heath. 
The wear of B, in measuring 38 chains, appeared (vid. art. 
iii.) to be 1% divisions of the micrometer head = l ~ = 
0,00673 inches : and the wear of A w r as 9,7 divisions = = 
°,°373 inches. 
Then, from the excess of A above Inches . IncheS{ 
100 feet, namely, - - 0,11425, and of B 0,05825 
subtract half the wear - 0,01865 0,00336 
0,0956 0,05489 
Ft. In. 
And we get the lengths of the chains' 
in the temperature of 54 deg. before 
they were used in the measurement, 
namely. 
A = 100 -f >0956, and 
B == 100 -f ,05489,^ 
lengths used in the re- 
duction of the base. 
art. viii. Method of fixing the Iron Cannon at the Extremities 
of the Base on Hounslow Heath, 1791. 
As the pipes were found in a very decayed state, and it be- 
came certain, were they suffered to remain as the termini 
that in a few years the points marking the extremities of the 
base would be lost, it became necessary to re-establish them 
in a more permanent manner. Amongst the various means 
mdccxcv. 3 L 
