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from “fome of them, where navigations are near, are very 
¢onfiderabl 
le. o 
_ GRINDERS, Morares dentes. See Molares under the 
article CRANIUM. 
GRINDING, Trirvuration, the a&t of breaking or 
eomminuting a folid body, and reducing it into powder, 
dutt, flour, farina, or the like, 
rinding is one of the f{pecies of diffolution. The paint- 
ers’ colours are ground on a marble or porphyry, either with 
oil or gum-water, 
- Some late phyficians contend, that digeftion is performed 
by grinding the food in the ftomach. See Digestion. 
Grinpine is alfo ufed for rubbing or wearing off the ir- 
regular or otherwife redundant parts of the furface of a 
body, and reducing it to the deitined figure, whether that 
be flat, concave, or the like. 
The grinding of glaffes is a confiderable art, and, as fuch, 
neceflarily requires to be here infifted on ; efpecially that of 
optic glaffes. : 
ethod of grinding optic-glaffes——For convex glaffes, the 
firft obje&-is to provide a difh or bafon, within whofe cavity 
the glais is to be formed 
neave mirrors are to be here obferved. 
r 
co 
Ti 
ts] 
the fe; in order to this, lay them on clean ena 
purpofe: in o this, lay them on aper, and 
obferve what colours are projefied upon it; foe the lice 
are the colours of the glaffes. Always fet afide thofe 
the darker colours, and choofe the ighter; b 
whiteit and brighteft have ufually veins, and 
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inftrument, in this fituation, it eafily breaks in the dire¢tiot 
f the line drawn by the diamond. Having thus got 4 
fquare piece, defcribe two concentric circles upon it with a. 
pair of compaffes, one of whofe legs carries a diamond; the 
diameter of the inner circle to be equal to the breadth of 
the intended lens, and that of the outer fomewhat more ; 
and break off the corners, as above dire&ted, and the leffer 
inequalities take off on a grind-{tone, or the like xamine 
now whether the piece of glafs be every where equally” 
thick ; if it be not, reduce it to fuch equality by grinding 
it on an iron plate with fand and water. y, glue er 
cement the glafs thus prepared to a wooden handle NM O,._ 
Jig. 4- with a cement made of pitch and a fourth part of © 
refin, or one part of wax and cleven of colophony ¢ care be= 
ing taken that the bafe or bottom of the handle N O be: 
equal to the glafs, and that the centre of the glafs and 
handle meet together. as thofe ufed for 
on it 
fometimes that, to prevent the form of the bafon from being. 
fine fand ufed in hour-olaffes may be of fervice, 
ufed as before ; Fereit i i 
oo much worn, an 
oe 
giaffes. | 
Fix the dith HI, fg. 5. on a horizontal table ; exaily — 
over its centre let the aperture D be, th fF 
AB: 
1 Now to grind 
the pe inftead of the dith, take hold of the 
wor b . 
en a finer powder of the fame, which-would be fifty 
aoe : esi : eme of : ; ere : 
ST ae teas aiht, nmiiey of the tr, aie apt toni Mowry et eufery of Our handled conde: Re eek aRa 
wes =e ; el egy uygens recom- of an hour; and laitly, eme of forty-five minutes, for a 
— whe boa > gs fe ia fo oTR or greenifh ; He- r of an ho he fame effe€t is had from powder 
— anh gh jdt bm to be tree from bub- of flints, broken in an iron mortar, mixed with water, and 
to ack cuca ae cata by piss S ‘t to the ftirred fometimes with a wooden fpatula, taking the powder 
ic the Sans a. tient ian. elt = As Se ees ae tN oe an in fome certain time to the bottom of the 
petri e pr ‘dow vellel, by decanting the water. What remains is to polifh 
Tf, inftead of ex Sek ong 
it muft be divided and cut with 
a diamond into {quares; and if it be too thick to bréak 
otherwife, you may do it by } ying it 
with cloth, in» fuch “as t e 
x > manner’ that fide’ 2 ee : 
‘fevered hang over the edge : for, bei “five et ta be 
make.nfe of plate-glay, it mutt be duided wel eee Oe 
ioe 
_ For amore particular account of the method of grinding 
and polifhing | glaffes for telefcopes, extraéted feta Mr. : 
Huygens and other authors, by Mr. Molyneux, and com- 
Prehending the method of making and polifhing she tools, 
Of choofing the glafs, of preparing the glaffes before they. 
can'be ground and polifhed, of grinding the glailes, and o 
: givu 
faid ftaff, and = 
