M. TECHNOLOGY. 485 



pointed, and is generally carelessly finished. The pencils 

 also absorb moisture from the air, and soon become soft and 

 smeary. 6 C, 18 75, 38. 



PREPARATION OF COPYING-INK PENCILS. 



According to Viedt, these pencils, or substitutes for ink, 

 can be prepared by making a thick paste of elutriated graph- 

 ite, finely powdered kaolin, and a very concentrated solution 

 of violet-blue aniline, soluble in water (or of any other aniline 

 color soluble in water), and pressing it, by means of a small 

 press, into sticks about four inches long and 0.1 to 0.15 of an 

 inch thick. When dry they are fit for immediate use. The 

 proper proportions of the ingredients can easily be ascertained 

 by trial ; and perhaps gum arable can be substituted for clay 

 as a cement. 13 C, July 1, 849. 



MARKING-INK UNAFFECTED BY CHLORINE. 



An ink for marking linen, etc., which is proof against 

 chlorine, may be prepared, according to Dr. Waltl, by dis- 

 solving: eig-ht ounces of extract of loo-wood in water, and 

 adding half an ounce of yellow chromate of potash and a 

 quarter of an ounce of chromate of copper, and evaporating 

 the mass until it begins to thicken, and finally adding half 

 an ounce of varnish. 26 (7, XL, 14. 



INERASIBLE BLACK INK. 



A so-called inerasible ink is prepared by Gaffard, of Paris, 

 of one part by weight of lampblack, twelve of potash water- 

 glass, of the consistency of sirup, one of ammonia water, and 

 thirty-eight of distilled water. 6 (7, XL, 8. 



cohausen's perigraph. 



Tracing the profiles of architectural objects, by obtaining 

 the co-ordinates of diff*erent points, rests so much upon the 

 delicacy of touch and sight of the individual that the results 

 are not accurate. A very simple instrument, devised by Co- 

 hausen and named a perigraph, eliminates this personal source 

 of error by rendering the operation a purely mechanical one. 

 It consists essentially of a lever, moving and sliding in such 

 a way on a pivot, on a board covered with drawing-paper, 

 that, when the board is held by the left hand from a ladder, 



