Page:Popular Science Monthly Volume 7.djvu/446

430 is usual in long land-lines, divided into lengths connected by telegraphic repeaters, the time of transmission will not be the same in both directions, and that the same effect would be produced in a submarine 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, though 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, surrroundedsurrounded [sic] 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 Company'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 R. H. Wyman, U. S. N., Hydrographer to the Navy Department, to turn his attention to the outfit of an expedition which should seek to determine with all possible accuracy the latitudes and longitudes of points connected by telegraph in that part of the world.

The authority of the Navy Department was readily obtained, and 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 tugboat 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.

The 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 with the zenith telescope, a modification working admirably in practice, and first suggested by Prof. C. S. Lyman, of