43 o THE POPULAR SCIENCE MONTHLY. 



is usual in long land-linos, divided into lengtlis connected by tele- 

 graphic repeaters, the time of transmission will not be the same in 

 both directions, and that the same effect would be produced in a sub- 

 marine cable having an imperfection or leak nearer one end than the 

 other. Experiments, however, by the Coast Survey on the long line 

 from Washington to San Francisco indicate that this objection, thougli 

 theoretically true, is of no practical importance. 



Upon land-lines the time-signals sent can be recorded directly on 

 the chronograph by putting it in the telegraphic circuit ; but, with 

 submarine cables, the electric impulse transmitted is not strong enough 

 to act upon the electro-magnets of the chronograph-pen. 



For telegraphing with weak impulses over submarine lines a very 

 beautiful device was invented by Sir William Thomson, and is now in 

 general use. 



To a delicately suspended magnet, surrrounded by a coil of fine 

 covered wire, a small mirror is attached. From this mirror a beam of 

 light from a lamp is reflected on a scale in a dark room. When no 

 currents are being sent over the line, this beam remains at rest; but, 

 when, at the sending station, either of two keys is pressed, a positive 

 or negative current, as the case may be, is sent through the cable, and 

 through the coil surrounding the magnet, causing the magnet with its 

 mirror to turn and to deflect the ray of light to the right or left. 



When the signal arrives and is perceived, the observer touches his 

 chronograph-key, thus recording the time of its arrival. 



The completion of the West India and Panama Telegraph Com- 

 pany's cable in 1873, and the certainty that serious errors existed in 

 the geographical positions of many places in the West Indies and 

 South America, caused Commodore H. H. Wyman, U. S. IST., Hydrog- 

 rapher to the Navy Department, to turn his attention to the outfit of 

 an expedition which should seek to determine with all possible accu- 

 racy the latitudes and longitudes of points connected by telegraph in 

 that part of the world. 



The authority of tlie Navy Department was readily obtained, aiul 

 the necessary preparations were commenced in the spring of 1873. 



In order that the work might be accomplished with economy, as 

 small a vessel as possible was desirable, the Fortune, a strong iron tug- 

 boat of 300 tons, being selected and prepared. Although this little 

 vessel carried the officers and men of the expedition safely, she was 

 found to be too small to encounter heavy weather at sea with any 

 degree of comfort. 



Tlie astronomical outfit was superintended by Mr. J. A. Rogers, 

 of the Hydrographic Office, and was in all respects satisfactory. 



The telescopes used were constructed at the repair-shop of the 

 Hydrographic Office for the purpose, and were a combination of the 

 transit instrument Avith the zenith telescope, a modification working 

 admirably in practice, and first suggested by Prof. C. S. Lyman, of 



