Meafuremcnt of a Bafe on Hoimflow-Heath. 441 
Befcription of the Gifs Rods , ultimately made ufe df to deter- 
mine 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 conftru&ion as meafuring rods. Notwithfianding 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 objedt in the axis of the bore at the other 
end. 
The nature and conftrudtion of the glafs rods, whereof 
three were finifhed for the operation, will be belt conceived by 
eonfidering, with care and attention, the plans and elevations 
of them, in whole or in part, to different fcales in tab. XIX. ; 
where likewife may be feen, plans and fections of the ends of 
the tubes, in their real dimenfions, for the better underffanding 
the feveral parts of the apparatus placed therein. 
The cafe containing the tube, and which ferves to keep it 
from bending in its original ftraight pofition, is every where of 
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 (ides being half an inch, 
and the top and bottom three-eighths in thicknefs. Thefe 
Haft 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 fattened by 
two rows of wood fcrews on each fide, to the top and bottom 
M m m z * refpeo 
* 
