Meafuremeni of a Bafe on Ho.unflow-Heath. 409 



of the horizoatai clamp, according to the fide on which the 

 rod lies that is to be moved on into coincidence ; and the other 



■: ihews it actually in its place, with the clamp itielf detached in 

 elevation along-lidc of it. Thus from the plan it may be 

 perceived, that the firfl, or adjufled rod, lies towards the far- 

 ther fide of the table, and is there fecured by the vertical 

 clamp. The fecond, or moveable rod, lies on the hither fide, 

 and therefore the horizontal clamp is placed in the farther 

 groove, where it is firmly pinched by the nut underneath. The 

 rod has been brought to coincidence by working with the tv/o 

 milled-headed fcrews agalnft the oppofite fides of the crofs foot. 

 This apparatus, although perfectly good in theory, was found 

 to be much too confined in its nature to anfvver well in prac- 



'tice, requiring the flands to be placed with a degree of preci- 

 fion, which could not be efFe6led in the field without great lofs 

 of time ; and this was the real caufe, as will be feen hereafter, 

 that the meafurement by coincidences with the deal rods was 

 given up, and that by conta6ls adhered to. 



Towards the left-hand of tab. XVIII. the plan of one of 

 the fquare tables is reprefented with the ends of the f:cond and 

 third rods upon it in conta£t. In this operation it will h^ 

 perceived, that only one crofs-fcot.of each rod could no^v. rcfl 

 on and be clamped to the ftand, the tables having beeiilnad- 

 vertently cut too fmall to admit of both ; and although tliis 

 has the appearance of iraperfeclion, yet no incoaveniency 

 whatever waj. found to refult from it in pradice^ experience 

 having (hewn, that the clamping of either end fufiiceci to keep 

 the„rod fleady. i\long-fide of the table, the vertical clamp, 

 being that now folely made ufe of, is likewife reprefented in 



elevation. 



Hhh2 On 



