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the party may for a while lose sight of called the three-point problem. From the all signal points. The forward rodman shape of the triangle, the worker can selects a slation, and that station is oc- make a better guess of the proper locacupied by the party and instrument. The tion of the true point, which he does, and rear rodman occupies the station just left, orients all over again. The triangle of and the table is oriented upon his rod. error this time will be smaller. A skillThe forward rodman again goes ahead, ful worker can get the true point in two selects a new station, the party moves, and again orients upon the rear rodman, who is standing over the station just left. Two miles are sometimes traveled in this way, without sight of a signal; but more than that, it is not wise to attempt. Of course, as soon as any known signal is in sight, the orientation is checked by its means; and if it be much out of the way, the charting is either done over or corrected by other means.
It not infrequently happens in river work that a bend or a bay may be out of sight of signals. In this case, a temporary signal called a plane-table signal is erected, which is in sight of the other signals, and which can be
Rodman ON STATION, with TELEMETER Rod. seen from where the are the The divisions on the rod, read between the cross-hairs on the telescope, indicate
distance between rod and telescope. work is to be done. The point is selected, the table set up, and all or three trials with surprising accuracy. made ready for work as before. But the Once the true point is determined on the table cannot be oriented, because there is chart, that spot upon the land is as effectno point upon the chart corresponding to ive for work as if it were a triangulation the location of the table. The man in signal. A temporary signal is erected charge, however, has a pretty definite idea over this spot, and work done from it exas to where he is, and makes a tentative actly as if it were a permanent structure. point upon the chart with his pencil, pro- The accompanying pictures will give ceeding then to orient the table with that one a better idea of what a signal is than as a base. Of course this operation is not any written description. A permanent accurate, and, as a result, the lines from signal is a triangular structure of heavy other signals form a triangle around the timber, anchored to the land by means point he has selected. This triangle is of heavy stakes driven deeply into the called the triangle of error, and the de- ground, and having a flagstaff projecting termination of the correct position is upward from its center. The sides of the triangle are boarded together for se- from the tender in which the instruments curity, and the peg and bottle placed be- are conveyed from the launch to the neath it as already described. A tempo shore. Nevertheless, the parties are all rary signal is usually made of a stripped imbued with the idea that their work tree, braced with poles and anchored ; and must be done well though the heavens bears upon its end a flag, and perhaps a fall; and indeed, if it were not for the lozenge, made of cloth and tacked down. painstaking care in this work, who knows It is very important that a signal be per- what wrecks and disasters might rependicular. If it is not, it will form an sult? Your mariner accepts without angle with the cross-hair in the telescope, question the information furnished him
on a chart, and runs his craft accordingly. If any glaring errors have been made, lives and property may be sacrificed. The writer has had some experience with the work, and met many men engaged in it, and he has yet to see one who did not take the work with the necessary seriousness, or who counted any discomfort or hardship as of any moment whatever against the proper execution of the work.
When the field work of a chart is finished, it goes to the home office, where it passes through a great deal which is of much interest before it is finally issued in the form of a chart. But that work, and also the hydrographic work, are separate stories, of surpassing interest of themselves, but not belonging, properly speaking, to a consideration of plane
What we have told is but an outline.
There is more—much more—to the full and not be as accurate (although of use of a plane table than we have indicourse the hairs can be turned to any
cated, but the present story covers the angle) as if it were straight. So, when a
most common practice and the principal signal is erected, it is plumbed before be
use of the instrument. To attempt, howing fastened ; the man in charge holds a
ever, to write a plane-table manual in a plumb line on it, and directs those who
popular article of this length, would be are putting it up how much to one side
as absurd as to try to explain the science or another it must go before being fasof algebra in five pages of a magazine. tened.
The plane table is unique among surAccuracy Never Sacrificed through veying methods, inasmuch as it draws Hardship
the map from nature's model absolutely A plane-table party often endures con- without notes, all work being done on siderable hardship in its work. Mosqui- the sheet in the field. With it, all is suftoes are thick along certain parts of the ficiently accurate at the scale at which coast and rivers, and bother the workers it is employed, and its use has been —who need their hands-in no incon- brought to a high state of perfection in siderable degree. The weather is often this country. Without it, the making of either very hot, under the broiling sun, charts would be a much slower and more or, later in the year, very cold. There is laborious process than is at present the a good deal of getting wet in the work case; and the development of the records —from high seas for a small launch, and of our water lines would be correspondfrom wading in along a shallow bottom ingly retarded.
How These Arteries of Submarine Communication are Constructed,
Laid, Operated, and Repaired
MONG the agencies for the elec- in 1852; and in 1858 the Atlantic Ocean trical transmission of intelli- was made the home of one of these margence, the submarine cable is velous aids to human enterprise. The
unique, playing a far more im- first Channel cable was hauled up by portant part in the economy of commerce a Frenchman on his anchor; and, thinkand diplomacy than the land line and telephone. When, in 1844, Morse demonstrated the feasibility of telegraphing overland, the inevitable corollary was the devising of some means whereby rivers, lakes, and, ultimately, seas and oceans, could be bridged. Scientists quickly conceived the idea of submerging a wire adequately insulated, though the practical difficulties in the way of securing an effective insulator took years to overcome. l'ltimately, a copper wire, covered with gutta-percha, was laid in Portsmouth Harbor in 1846. Dover and Calais were connected below the bed of the English Channel in 1850; Harwich and The Hague were joined under the North Sea
ROUTES OF PACIFIC CABLES.
ing it a sea serpent, he severed it with an axe. Many Newfoundland fishermen thought the first Atlantic cable would be an excellent thing to utilize for mooring their boats.
The development of submarine cables has been remarkable, both in their widespread use and in their enhanced efficiency. The North Atlantic is now spanned by fifteen cables, and the Pacific by two; and other seas have an ample quota besides. The total is 1,764 distinct cables, with an aggregate mileage of 211,000, interlacing not alone the continents but the most widely separated insular areas of the world, until only the remote and unpeopled Arctic and Antarctic zones are without this potent accessory of modern civilization. Even as this is written, it is proposed to lay a cable between Iceland and the Orkneys, there connecting with the British Postal Telegraph system of cables and land lines, so that, by an extension later to Greenland, or by the use of the wireless medium, it may be possible to follow the - daily movements of future seekers for the North Pole.
How a Cable is Made The cable is merely the land line completely insulated for its whole length (the water being so potent a conductor of electric force), and with its metal core of a substance affording the highest “conductivity," as no repeating stations are possible between land and land. Copper is the best known conductor for cables, and the electrolytic quality is almost absolutely pure. It is now being used for land lines, its great cost formerly preventing this. Gutta-percha, in the same way, is the best insulator-not absolute indeed, but, according to some authority, having a conductivity so small that "it may be said to stand in the same relation to copper as the rate of a body moving through one foot in 6,700 years is to the velocity of light, which attains 186,000 miles in a second.”
In the early days of cable-laying, these problems had all to be solved by the electricians of the period—Faraday, Thomson, Whitehouse, Wheatstone, Muirhead, and others. The "velocity of propagation" of the signals involved the efficiency
and economy of the cable. Faraday de- years ago I sold the system to the termined the electrical capacity of the Western Union for $30,000, and spent cable, the unit in which that is now meas- the whole of it in experiments for ured being ever since termed the "farad.” making a wire to carry six mesThomson found a mathematical defini- sages instead of four. I did not tion for the capacity and its dependence succeed; so that, financially, I am worse on the dimensions and insulating mate- off than I would have been had I never rial. Whitehouse proved that large bat- invented the quadruplex system at all.” tery power was not needed for long ca- Cables are made chiefly in England, bles, as the Irish pilot proved the pres- though France, Germany, and Italy are ence of rocks-by working the first Atlantic cable with such an electrical charge that he forced it through the insulation and destroyed that medium, rendering the cable valueless. Wheatstone invented the automatic transmitter ; Thomson, the siphon recorder; and Muirhead, the duplexing system. Edison concurrently invented a quadruplex method of land telegraphy, of which, semihumorously, he says himself: “Twenty-five
LANDING-PLACE OF FRENCH CABLE, ST. PIERRE.