M. TECHNOLOGY. 531 



object-glasses by 0.6 of an inch, he succeeded in photograph- 

 ing a divided scale placed at over one hundred feet distance; 

 the lines were quite distinct. 12 A, X., 76. 



SUBMARINE PHOTOGRAPHY. 



Dr. Neumeyer's plan of a photographic apparatus, design- 

 ed for the determination of the temperature and currents at 

 the bottom of the sea, is described as follows : A copper box, 

 hermetically sealed, is provided with an exterior appendix 

 made like a rudder. A mercurial thermometer and a com- 

 pass are in the interior, each in a glass receptacle, in which 

 are admitted traces of nitrogen gas. A small electrical bat- 

 tery completes the apparatus. When the apparatus is low- 

 ered into the sea, attached to a sounding-line, the action of 

 the current on the rudder indicates the set of the flow by 

 the relative position of the compass-needle and rudder. The 

 temperature is recorded by the thermometer. To fix these 

 indications, a piece of photographic paper is suitably dis- 

 posed near the glass case containing the instruments. At 

 the proper moment a current of electricity is established 

 through the gas in the receptacle, causing an intense violet 

 light, capable of acting chemically upon the paper for a suf- 

 ficient length of time to allow of impressing photographic- 

 ally thereon the images of the compass-needle and of the 

 mercury column. The time required for this register is said 

 to be three minutes, after which the instrument is hoisted, 

 the paper removed, and replaced by another for a second 

 observation. 



CONVERSION OF LITHOGRAPHIC INTO LETTER-PRESS PLATES. 



A perfectly smooth and polished zinc plate is first coat- 

 ed with weak phosphoric acid, then well wiped off, quickly 

 placed over an alcohol flame, and rubbed dry with a woolen 

 cloth. An impression on paper from the lithographic orig- 

 inal in a moist condition is placed upon the zinc, and passed 

 tli rough the press several times. It is then treated as an 

 ordinary lithographic stone, except that varnish is employed 

 instead of oil of turpentine. The plate is next wiped off 

 and dried, and the drawing dusted with finely powdered 

 colophony, every particle of the latter being carefully re- 

 moved from the free portions of the plate. It is then very 



