Graduation of Agronomical Injlruments. 45 
except at the terminations, which will in a great meafure protect 
it from both alteration of moifture, and of heat from the body 
of the artift, during the operation. 
It will be likewife proper to have a lever, or feme equivalent 
contrivance, to bring the dividing plate forward ; that after 
lifting the little cylinders out of the divifions, and refting 
them upon the tops of the teeth, they may be brought gently 
forward with an equal drag, and ultimately fnap in between 
the teeth, by the ftrength of the fpring commanding the index; 
by this means the drag of the fri&ion of the whole will be 
conftantly the fame way. 
Conclufon . 
Now, if, as it has been fhewn, a quadrant of any radius may 
be read off to the 400odth part of an inch, then this quantity 
upon a radius of three feet will not be fo much as 1 § fecond 1 
and as the whole of the procefs is carried on by contact, in 
which a greater error than that of a 6o,ooodth part of an 
inch cannot be admitted in any Angle operation, I fhould 
affuredly expedfc a three- feet quadrant, fo divided, to be true in 
its divifions, and read off to at moft two feconds. 
But, after all, in an inftrument like this, I fihould exped the 
greateft fource of error to be in the want of perfed coincidence 
of the center of the divifions with the aftual center upon 
which the index revolves ; and therefore, that if, inftead of a 
quadrant of three-feet radius, a complete circle of five feet 
diameter was divided, and its divifions read off from the two 
oppofite points (taking the mean), then the errors of the 
center will be wholly avoided. For this reafon, I am very 
dearly of opinion, that the fagacious propofition of MivRamsf* 
DENp . 
