good one, when it is an Inch or two out of it. And I have a 
good one by me atprefent, of ^6, foot, that will b?.rc an Aper-' 
ture^ if Saturn or the "^oon in thQtwiltght, be look'd on with it, 
of 3 ^ Inches oveT,and yet the thickeft part of the Glafs is a great 
way out of the middle. And Imufttake the liberty to doubt, 
whether ever my Animcidverfor faw a long Glafs^that was other- 
wife; as he might prefentlylatisfie himfelf by a way I could (hew 
him^ifhedid not know it) whereby the difference of the thick- 
ne^s of the fides might be found to the hundreth part of a Line. 
As to the exceeding exadtnefs of the Figure of Long Objeil* 
cUaJpSj'tis not doubted, but that it is a matter difficult enough to 
be attained any way: but yet^I think.much cafier by Enj^ine,ihm 
by Hand and of all Engines, I conceive, none more plain and 
fimp!e,than thatof aM^w^'n/. And iox m2ik\ng fpherisal Clajfis 
by anEngine^l am'apt to thinkjtherc hardly can be any way more 
plain, and more exadt, than that which I have defcribed 5 where^ 
in there is no other motion, than that of two [uch Mandrils^ 
which may be made of (ufficient ftrength, lcngth,and exadtnefs, 
to perform abundantly much more, than lean believe poffible 
to be done otherwife than by chance, by a mans hands or 
ftrength unafEfted by an Engine, the motion and ftrength being 
much more certain and regular. I know very well, that in ma- 
king a do.footGlafs by the ftrength of the hand,in the common 
way,not one of ten that are wrought, will happen to be good, as 
I have been affured by Mr. Reeves ; who, I am apt to thinkj was 
the firft that made any good of that length. For the Figure of 
the Tool in that way is prefently vitiated by the working of the 
Glafs, and without much gaging will not do any thing confider- 
able. Befides,the ftrength of a man's hands, applied to it for the 
working and polifhing of it, is very uncqualj and the motions 
made,are very irregular ; but in the way,I have ventured to pro- 
pofe^by Mandrils^ihc longer the Glafs and Tool are wrought to- 
gether, the more exadt they feem to be and if all things be or- 
dered, as they fliould be, the very polifhing of the ^Glafs, does 
feemmoftof all reO:\Be the Figure. 
As to what he objedlsjthat the Tool docs only touchthe Glafs 
in a n4athematieal Circle ^ that is true,perhaps, at firft, but before 
the Glafs is wrought down to its true Figure ^ihcEdge ©f the Tool 
