158 ANNUAL OF SCIENTIFIC DISCOVERY. 



being made to scale, so as to represent the measured distance of 

 the two places of observation, it follows that the position of the 

 two needles will indicate the two lines of sight of the two tele- 

 scopes both fixed upon the same distant object, and the point 

 where the two needles cross each other (one of the needles being 

 slightly below the other) will correspond to the exact position of 

 the* distant object. If the latter is in motion, and the two observ- 

 ers follow its movements so as to keep it constantly in si^ht, the 

 two needle* will constantly change their position, and their point 

 of intersection will make the same movements upon the map, on 

 a small scale, as the distant object makes in reality ; the move- 

 ments of the object and those of the point of intersection of the 

 two needles being simultaneous. For purposes of warfare 

 there are several applications of this instrument, which will 

 readity suggest themselves ; but similar instruments may be used 

 with advantage for purposes of general surveys of land, and for 

 similar operations where they are not unlikely to effect some con- 

 siderable saving of time, if properly employed. Engineering. 



EARTH CIRCUIT IN TELEGRAPHY. 



The failure of the earth circuit of a short telegraphic line in the 

 Pewabic copper mine, Lake Superior, is interesting from a prac- 

 tical point of view. The wire used was a one-sixteenth inch cop- 

 per wire, wound in the same manner as waterproof fuse, the wire 

 taking the place of the powder. To the surprise of all, no signals 

 could be transmitted through the line. The end of the wire un- 

 derground was put into a hole drilled into the rock and tamped 

 in; a bed of earth was then made, and lastly a pool of water 

 tried, but all to no effect. Above ground the line worked well 

 enough. 



Though the earth, generally speaking, will conduct electricity, 

 some substances, of which any specific portion of the earth may be 

 composed, will not conduct it; for example, dry sand and dry 

 freestone rock will not, and quartz rock will not any more than 

 glass ; dry earth will not, as is recognized by all telegraph con- 

 structors, who bury the earth-plates deep in damp earth. In this 

 case an attempt was made to form an earth circuit in non-con- 

 ducting material. The end of the wire in the mine was tamped 

 into the solid rock, probably quartz, which would be about the 

 same as tamping it into a glass bottle, filled with earth or water. 

 The chances of electric communication would be still less, if the 

 wire was not perfectly insulated in its whole length. The remedy 

 would be to make a return circuit of insulated wire. Mechanics* 

 Magazine, 1867. 



A NEW SAND BATTERY. 



Father Secchi, of Rome, in a recent number of the "Labora- 

 tory," described a new, simple, and useful galvanic battery. He 

 takes a piece of thin sheet copper, 8 inches square, and cuts on 

 one side 6 notches 1 inches deep, so that 6 points are left. 

 The points are alternately bent in horizontally, and the sheet is 



