Meafurcment of a Bafe on HounOow-Hcath. 441 



DefcnptJon of the Ghfs Rods, ultimately made ufe of to deter- 

 7nine the length of the Bafe. Tab. XIX. 



It has been already mentioned, that the week of rainy wea- 

 ther in the end of July was employed in providing the glafs 

 tubes, and in concerting matters with Mr. Ramsden, relative to 

 their conftrudion as meafuring rods. Notwithfknding their 

 great length, they were found to be fo flraight that, when laid 

 on a table, the eye, placed at one end looking through them^ 

 could fee any fmall obje6i: in the axis of the bore at the other 

 end. 



The nature and conftruiflion of the glafs rods, whereof 

 three were finifhed for the operation, will be befl conceived by 

 confidering, with care and attention, the plans and elevations 

 of them, in whole or in part, to different fcales in tab. XIX. ; 

 where likewlfe may be feen, plans and feclions of the ends of 

 the tubes, in their real dimenfions, for the better nnderflanding 

 the ieveral parts of the apparatus placed therein* 



The cafe containing the tube, and w^hich ferves to keep it 

 from bending in its original flraight pofition, is every whereof 

 the depth of eight inches, of the fame width in the middle, 

 and tapers from thence, in a curvilinear manner, towards 

 each end, where it is only two inches and a quarter broad. It is 

 made of clean white deal, the two fides being half an inch, 

 and the top and bottom three-eighths in thicknefs. Thefe 

 laft are placed in grooves fitted to receive them, about half an 

 inch from the upper and lower edges of the fides, which bend- 

 ing eafily, and applying clofely, are then firmly faftened by 

 two rows of wood fcrews on each fide, to the top and bottom 



M m m 2 J refpec* 



