The. Pritt met. 
befids fuuch ‘determinate dimenfions , are by certain inconvensences rendred 
unufeful for it. willbe exceeding difficult to make and manage aZube 
 phowe an hundred footiong, and it will be as difficult to inlighten an 
bee lef then an bundred part of an inch diftant from the Objet Glas. 
“TL bave mot os yet made any attempts of that kind, though I knowtwo or 
three wayes, which, as far asT have yet confidered, feem very probablevand 
mayinvite me to make a tryal as Joon as Ihave an opportunity, of whichE 
may hereafter perbaps acquaint the world. Jn the Interim, I foal defcribe 
the Infirument Ieven now at by which the refra€tion of all L Rinds 4 
of Liquors may be moft exattly meafur dy, thereby to give the curious an 
| Gpportinity of making what further tryals of that kind tbey fhall think, 
requifite to any of their intended tryals ; and to let them fee that the laws 
of Refrattion are not only notional. d ys 
The Inftrument confifted of five Rulers , or sant pieces placed together, — 
after the manner expreft in the fecond: Figure of the firft Scheme, where 
AB denotes aftraight piece of wood about fix foot and two inches long, 
about three inches over ,’ and ‘an inch and half thick , on the back fide of 
which was hing afmall plummet by a line ftretchr from top to bottom, by 
which this piece was fet exattly upright,and fo'very firmly fixt 5 in themi 
dfe of this was made a hole orcenter, into which one end of a hollow éshs 
lindrical brafs Box CC, fafhion’d as I fhall by and by defcribe, wasiplac'd; 
and could ver ; -and truly - mov'd to and fro + “the other end:of this 
: being put into, , ahole made in afmall arm DD; into 
this box was fafthed thet Ruler EF, about three foot and three or four 
inches long, and at three foot from the above'mention’d CentersPP was 
a ‘hole E, cut through, and crof'd’with two fmall threads, ‘and attheendof 
_ it was fixt aimall fight’G, and on the back fide of it was fixt a fmall Arm. By 
with a Screw to fix it inany place on the Ruler LM; thisRuler LM was 
moy'd onthe Center B ( which was exactly three foot diftance from the 
middle Center P!) anda lite drawn throtigh the middie of it LM was 
divided by a Line,of cords into fome fen ty degrees,and each degree was fab- 
diyided into minutes, fothat. poring he e crofs of the threads in E upon any 
part of this divided line ; 7 prefenitly ew what Angle the two Rules AB 
and\E.F madé with each other, ‘and. by turning the Screw. inH, I could fix 
them.in.an neha The other Ruler allo Rs RS was made.much after the 
fame: ah ‘Gn 4 it was not fixt to the hollow cylindrical Box, but,by means 
of ‘two {mall ‘Armes or Ears, it mov'd on the Centers of it ; thisalfo, 
-means of thierofs threads in the hole $,\ and by a Screw. in K, could ‘be 
d on any divifion of another line of co sof 
NO. And fo! xy that means, the Angle made: by the two Ruilérs, AB and 
RLS, ‘was alfo known. The Brafs box CC in the middle was fhap'd. very 
much like the Figure X, that is, it wasa cylindrical Box ftopp'd clofe at ei- 
ther mom of whicha ite both of the fides ‘and bottomes was cut out, fo 
that 
