GRINDING. 
fpr the oftener thele are ground together, you will be:the . wrong figure for, befides-faving the trouble in grindingy 
re {ure of having your figure true. Phil, Tranf. vol. lyiii, it is beft, on many accounts, to have the f{peculum caft of a. 
p- 507, &c. ; ; so se true figure; and for this reafon it is beft to cai it froma 
Method of Grinding, Sc. the fpecula of refleding telefcopes.. hard pewter pattern, and not from a wooden one, as founders : 
men is that ufually caft. ; 
propofed by Mefirs, Molyneux and Hadley, and publifhed Tor the compofition of the metal and manner of cal. 
in’ Dr. Smith’s Optics; this has been ately im- it, fee Specutum. When the metal is cait, the next bufinefs | 
prove confiderable degree, by Mr. Mudge, is to grind and polifh it, for which, fays Mr. Mudge, four 
whofe directions, founded on accurate experiments, and tools are all that are nece ary, viz. 1.. Ther ugh-grinder, 
practifed with fuccefs in the conftru&tion of telefcopes, wé_ for working all the rough face of the metal: this is beft 
adopt wherever they differ from thofe that occur in the made o lead, ftiffened with about a fifth or fixth part of tin 5, 
book jult cited. For making the gages, take a long pole it fhould be at leaft a third more in diameter than the metal. 
of fir, deal, or any wood, of a little more than double the which isto be ground ; and for one of any fize, net lefs than: 
length of the inftrument intended, and {trike through each aninch thick. — It. may be cemented on a block of,wood, in 
order to raife it higher from the bench. J 
Up perpendicularly againft a wall; then take two pieces of — This leaden tool being caft, muft be fixed in the lathe, and. 
hi turned as true as poffible by the gage to the figure of the in=, 
pence, which may be about an inch and a half broad, and tended fpeculum, making a hole in the middle, as a lodg-. 
i ment for the emery, of about an inch diameter, for a metal. 
of fcurinches : when this is dore, deep grooves muft be cut. 
acrofs its furface with a graver, in the manner reprefented in 
thefe flrongly with rivets between two thin pieces of wain- Plate V.. Optics, fg. 63 thefe grooves will ferve. to lodge, 
cot, fo.that a little more than a quarter of an inch in the. the emery, and by their means the tool will cut much falters. 
breadth may ftand out from between the boards. Then Any kind of low handle, fixed on the back of the metal with 
fix up thefe pieces horizontally againft the wall under your foft cement, will be fufficient ; but it fhould cover two-thirds. 
pole, and therewith, as with a beam-compafs, ftrike an arch of its-back, to prevent its bendin 
open each of them; then file each of them with a fmooth 2. 
The next tcol is the Pind brafs-grinder, on which’ 
: : the metal is to receive its {pherical figure. In order to form: 
other a concave arch of the fame circle. Thefe braffes are thi ; 
the gages to keep the {peculum, and the tools on which it is at moft a fixth part 
Phis g turned, in the manner. hereafter to be defcribed. ‘This being doney. 
ake ome common pewter, and mix with it about. one-tenth it mutt be fixed and turned in the lathe with 
of regulus of antimony, and with that wooden pattern, caft 
one of this pewter, which will be confiderably harder than This tool muft have a hole, fomewhat lefs than that in the 
common pewter. Let this pewter pattern be truly turned metal to be worked upon it, in the middle, quite through. 
¥ means of the gages before-men- to the bottom. When it ié finifhed off in the lathe, its: 
cai! Sins fpecula themfelyes ; and diameter fhould be one-eighth wider than the metal. ie 
aig 
nch broader perfeé& the {pherical figure, and to give the metal a fine 
J ee S ‘ . Pe = 
wider ev ry way than the fpeculum intended. In each fla: c ee ahi) poate 
ir fhould he the thickets. at ah af ca neach flafle as poffible ; it fhould be then feraped, 
t 
= 
& 
S 
2 
@ 
z 
> 
2 
ct 
zs 
tes fhould be between the hones and th 
afks dry each other with fine emery and water, 
- the fame figure as nearly 
a wathed emery mult be p 
In the fas Bag Pees hours, or near a very gentle fire, o' 
wie they will warp, and give the Speculum, when ¢ 
Bs aes 
* 
