Trigonometrical Survey. 557 



belonging to the Royal Society, pickets, iron heads, and a new 

 set of coffers, were sent to Somerton, after Mr. Gardner had 

 been furnished with the means of proceeding with the survey 

 before spoken of. 



The measurement was begun in July, and finished in August; 

 in the course of which, very little interruption arose from any 

 inclemency of weather. It is unnecessary to enter minutely into 

 a description of the difficulties which arose from the frequent 

 intervention of ditches ; let it suffice to observe, that, possessed 

 of the 50-feet chain, these were rendered less material than they 

 would otherwise have been. 



When we arrived at that point which ends with the 114th 

 chain, an offset was taken, and 19 chains measured, in a direc- 

 tion perfectly parallel to that of the base, at the extremity of 

 which we returned into the base itself, and continued the mea- 

 surement. This interruption proceeded from an accidental and 

 unforeseen circumstance ; a great ditch having been excavated 

 in a direction coincident with that of the base, while the mea- 

 surement was going on at the upper end of it. This, however, 

 cannot be the means of introducing any sensible inaccuracy; 

 for, to proceed in this matter correctly, when it became neces- 

 sary to take an offset, a silver wire was let fall from the register 

 head, having a plummet, under the point of which a small dot 

 was made, on a stake driven firmly into the ground. The great 

 theodolite was then placed over the stake, and the instrument 

 accurately adjusted over the dot. A diaphragm, whose aperture 

 was -§- an inch, was then put over the object-glass of the transit 

 telescope, which was afterwards directed towards the staff at 

 Lugshorn Corner, and then moved round, till it exactly made a 

 right angle with the base. The telescope being sufficiently 



mdccc. 4C 



