Miscellanies. 1 59 



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 

 rock, 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. SILLIMAJV. 



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. 

 One 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- 

 tus, where, I presume, it may now be seen. This instrument per- 

 forms the office of scale, dividers, parallel ruler, and protractor; and 

 it 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 protractor, and the scale 

 open 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 

 a graduated nonius, and the graduadons fitted to the decimal divis- 

 ions of an inch on the scale. A prick-point fixed to the under side 

 of a 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. 



