good one, when it is an Inch or two out of it. And I have a 
good one by me at 'prefen t, of 36. foot, that will be?.r an Aper- 
ture 9 it Saturn or the Moon in th q twilight, be look’d on with it, 
of 5 \ Inches over, and yet the thickeft part of the Glafs is a great 
way out of the middle. And I muff take the liberty to doubt, 
whether ever my Animadverfor faw a long Glafs, tha: was other- 
wife; as he might prefentlylatisfie himfelf by a way Icould ffiew 
him’fif.he did not know it) whereby the difference of the thick- 
nels of the (ides might be found to the hundreth part of a Line. 
As to the exceeding exadfnefs of the Figure of Long Ohje$~ 
GlajfpSi’ti s not doubted, but that it is a matter difficult enough to 
be attained any way: but yet, I thinkjinuch eafier by Engines han 
bjHandi and of all Engines, I conceive, none more plain and 
finiple,than that of a Mandril. And for makin g fpkerical Glajjks 
by zwEnginefl am apt to think,there hardly can be any way more 
plain, and more exatft, than that which I have defcribed ; where* 
in there is no other motion, than that of two inch Mandrils 0 
which may be made of fufficient ftrength, length, and exadtnefs, 
to perform abundantly much more, than lean believe poffible 
to be done otherwife than by chance, by a. man’s hands or 
ftrength unaffifted 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, l am apt to think 5 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. Belides,the ftrength of a man’s hands, applied to it for the 
working and poliffiing of it, is very unequal, and the motions 
made, are very irregular ; but in the way, I have ventured to pro- 
pofe,by Mandrils ^ the longer the Glafs and Tool are wrought to- 
gether, the more exaeft they feem to be, and if all things be or- 
dered, as they fhould be, the very poliffiing of the ^Glafs, does 
feem moft of all toreeftifieth e Figure. 
As to what he obje<fts,that the Tool does only touch the Glafs 
in a Mathematical Circle , that is true, perhaps, at firft, but before 
the Glafs is wrought down to its true Figure, theis^ of the Tool 
will 
