42 Mr. Smeaton’s Obfervations on the 
each fet of 1 20 divifions, may be fuppofed, if any, to be mended 
thereby; but as it may be incommodious to conftrudt a curved 
fcrew, of fuch a length and fize, in Hindley’s method, as 
would be fufficient for the purpofe, I would propofe to ufe two 
fcrews of brafs, cut from a cylinder in the way fet forth by 
Mr. Ramsden, each of which, with a very little grinding upon 
this large circumference, would lay hold of ten or twelve teeth 
together. I would place the two fcrews, that is, their middles, 
to be ninety divifions afunder ; of confequence, when one of 
the fcrews is between the 59th and the 60th, or between the 
119th and 120th divifion of each fet, the other will be in the 
middle of the fpace divided by the chaps only 
The threads of thefe fcrews I would advife to be cut a little 
taper, fo that as they grind in, they may fill the notches of the 
teeth; which alfo, by this means, will acquire a little tapering 
towards their extremities ; and by cutting the notches parallel* 
as I have mentioned, the true ground part will always be certain 
of being at the extremity. 
When the fcrews have been ufed in grinding till they are 
found to have the effeft of a perfectly equal and eafy rotation all 
round, and all the teeth reduced to a fenfible taper, and regular 
bearing, I would then totally remove the fcrews from the fquare 
block of wood, upon whofe upper face I fuppofe them to have 
been mounted; in like manner as I fuppofe the index and cutting 
frame to have been removed, to make room for the mounting of 
* The beft way of giving an equal motion to thofe two fcrews, feems to be by a 
detached axis, carrying two common fiat wheels ; one a&ing upon a like flat 
wheel, upon the axis of one fcrew, and the other, in the fame manner, upon 
the other ; and applying the pulley for communicating the power to the middle of 
the detached axis between the two wheels, the fpring or twill: will be equal both 
ways ; fo that in turning the contrary way round, they will ftill be in equal 
advance. 
4 
the 
