hoe 
Miscellanies. 159 
axil an inch and a fourth in diameter, without any crank. I have 
now used it constantly in this school, and the students have used it 
continually, for four years, with a substitute of the common adhesive 
substance, called diachylon by the druggists. This we mould into a 
form adapted to the mineral to be examined. A slender cylinder of 
it; with one end adhering to the broad end of the axis on one side of 
the center, arching so as to bring the angle to be measured in a line 
with the center, is required for a small crystal. A large crystal, or 
any crystal which adheres to a large mass of its gangue or embracing 
tock, requires a large piece of the diachylon, covering one side of the 
end of the axis. A mineral weighing several pounds, or only the 
fourth of a grain, may be thus fixed to the instrument, at the pleasure 
of the operator. 
Rensselaer School, March 3, 1831. 
9. Mapping Instrument. 
TO PROF. SILLIMAN. 
Several mapping or plotting instruments having been recently pro- 
Posed, and perhaps will be patented; please to permit the following 
Paragraph to go out in your next number. 
Any artist shall be welcome to the right of an invention of my 
own, of a mapping instrument, which received considerable attention 
about twenty years ago; but was never brought into extensive use. 
ne of the instruments was presented by myself to President Day, 
in the year 1816, when he was professor of natural philosophy. He 
told me afterwards that he had deposited it with the College appara- 
lus, where, I presume, it may now be seen. This instrument per- 
‘rms the office of scale, dividers, parallel ruler, and protractor ; and 
does not contain a joint. One may conceive of the construction of 
this instrument, by imagining one end of a six inch scale, brazed to 
the middle of the straight side of a common protracter, and the scale 
pen in the middle, half an inch in width, from the brazed end to 
near the other end—then imagining a slide to run in that opening with 
* graduated nonius, and the graduations fitted to the decimal divis- 
ons of an inch on the scale. A prick-point fixed to the under side 
ofa spring, attached to the slide at the end, towards the brazed end 
of the scale, completes the instrument. Yours respectfully, 
Amos Eaton. 
Troy, March 3, 1831, 
