4 7^ Tj&£ Account of a 
For the purpose of tracing out the line of the base, as Beacon 
Hill had a commanding view of almost the whole of it, the 
instrument was kept in the tent after the observations were 
finished : and at different times, when the air was sufficiently 
steady for the purpose, many points in the true direction were 
found by'bisecting the staff erected at Old Sarum, and mov- 
ing the transit in the vertical, whilst a person placed a camp- 
colour in the proper situation on the ground, by means of sig- 
nals which were made at Beacon Hill. 
As it appeared, when this spot was first selected for the mea- 
surement, that in the course of it there would be frequent ne- 
cessity for changing the directions of the hypotenuses, a brass 
bar, of a prismatic form, had been provided, by means of which, 
and a plumb-line, a new direction was easily taken. The me- 
thod of using them was as follows. 
A picket was driven into the ground close to the handle of 
the chain, having its top eight or ten inches above the place 
where the preceding hypotenuse was to terminate, one of the 
register-heads, with the bar, being screwed on it. The chain 
was then stretched, and the silver wire, or plumb-line, made 
to pass through the handle, whilst the slider was moved till the 
wire came upon the dart, marking by this means, the termina- 
tion of the hypotenuse. In order, however, to give a more 
perfect idea of this matter, a figure is given in Tab. XLV. 
where B is the bar, with the wire falling through the handle 
of the chain, one half of it being left out, for the purpose of 
showing its coincidence with the arrow on the handle. 
The experience which we had obtained in the measurement 
of the base on Hounslow Heath, led us to discover, that some 
of the methods we made use of to execute particular parts of 
it, might have been improved. One of them was, the means 
