Surveys, of coasts or countries must have well established starting points, and while the latitude of a place is comparatively easy to determine, the longitude, except when the telegraphic method is used, is attended with more or less uncertainty.
In 1873, Commodore R. H. Wyman, U. S. N. Hydrographer to the Bureau of Navigation, organized by permission of the Navy Department, an expedition for the telegraphic determination of longitude in the West Indies and Central America. The submarine cables of the West India and Panama Telegraph Co. had just been completed, extending from Key West through Havana and Santiago de Cuba, south to Jamaica and Aspinwall, and east through the Virgin and Windward Islands to the northeast coast of South America, thus affording admirable facilities for the accurate determination of many points. It had long been known that the longitudes of various points in the West Indies and in Central and South America, did not harmonize, there having been no systematic attempt to determine them with relation to each other or to a common base. Longitudes in the western part of the Caribbean Sea depended upon the position of the Morro lighthouse at Havana, which had been determined by occultations. Further to the eastward, positions depended upon that of Fort Christian at St. Thomas. This in its turn depended upon the observatory of Major Lang in the Island of Santa Cruz about forty miles distant. This position depended upon numerous observations of moon culminations and occultations. Martinique and Guadeloupe in the Windward Islands had been surveyed by French officers who based their positions upon longitudes derived from moon culminations. The absolute determination of these starting points would of course fix all points derived from them.
The U. S. Steamer Fortune was designated by the Navy Department for the conveyance of the expedition, and Lieut. Commander (now Commander), F. M. Green, U. S. N. was placed in charge. This officer had given great attention to the subject, was a practiced observer, and exceptionally well qualified for the position. The services of Mr. Miles Rock, a skillful astronomer and computer who is now chief of the boundary survey of Guatemala, were obtained as principal astronomical assistant. The breaking out in the autumn of 1873, of the trouble with Spain and Cuba, over the Virginius affair, delayed the expedition until the next year, but in November 1874, a start was made from Washington, and after a short stay in Kingston, Jamaica, Aspinwall was reached early in December. Mr. Rock with one set of instruments proceeded immediately to Panama, while Lieut. Commander Green remained in Aspinwall with the other. The outfit for each party consisted of:—first, a portable observatory. This was made of wood in sections, framework of ash, covered with tongued and grooved pine boards. The sections were connected when set up by iron knees and bolts. When packed it was not difficult to transport, and it could be put up, or taken down in an hour. When set up it was about eight feet square, with doors in all sides, and a shed roof. The roof was made in three sections, the middle one being hinged so that it could be raised for observing. These observatories proved to be very strong and serviceable. They remained in use for a number of years with occasional slight repairs, were transported many thousand miles and set up in a great number of places in Europe, Asia, North and South America. They were designed by Mr. J. A. Rogers, and constructed at the Washington Navy Yard. Upon arriving at a point where observations were to be made, after obtaining the necessary permits from the local authorities, a suitable location for the observatory was the first consideration. The essential requirements were, a clear view of the heavens in the meridian, firm ground, a spot secluded enough not to attract attention from inquisitive idlers, and proximity to the telegraph office, or end of the telegraph line. Such a spot being found and permission being obtained from the owner for its use, an approximate meridian line was laid out by compass, and the house set up with reference to it. Experience soon showed the advisability of making certain additions to the observatory not contemplated by the designer, but which added much to convenience and comfort. A foundation was made, of timbers about six inches square, mortised together at the ends which could be placed in position and leveled before the observatory was set up, rendering this operation much easier and giving greater stability. A floor was laid upon joists supported by this foundation. Shelves were put up at various points, affording resting places for tools and small instruments, while a table in one corner, supported the chronometer, and offered a convenient place for an assistant to record observations, etc.
The principal instrument used was the transit. Those furnished for the use of the expedition were designed by Mr. J. A. Rogers, and constructed under his supervision in the repair shop of the Hydrographic office. The object glasses, made by the Clarks at Cambridge, were of 2½ inches clear aperture with a focal length of thirty inches. The instruments were of the prismatic or "broken" form in which the eye-piece is at one end of the axis, and the light is reflected from the object glass to the eye by a prism placed at the junction of the telescope tube with the axis. The observer does not have to change the position of his eye, no matter what the zenith distance of the star may be. This renders observation much less fatiguing and conduces to accuracy. The eye-piece was furnished with the usual spider line reticle and also with a filar micrometer for the measurement of zenith distances for latitude. A vertical finding circle was on the eye-piece end of the axis, and the instrument was provided also with a horizontal circle, fourteen inches in diameter, graduated to ten seconds. Other necessary parts were the striding and zenith telescope levels, and the illuminating lamps. The ends of the axis were supported by Ys at the ends of a transverse arm which in its centre was screwed to the top of a vertical axis supported in a socket surmounting the tripod. This vertical axis was slightly conical in shape and accurately fitted into its socket. A screw was so placed underneath, that the axis, and with it the instrument, could be raised slightly, when it was easily revolved horizontally into any desired position, a reverse movement of the screw then lowered the axis into its seat, when the instrument was held firmly by the friction. For supporting the instrument there was used at first, a portable pier made in the shape of the frustrum of a cone, of strong oak staves, firmly bound with iron hoops, and when set up, filled with sand or earth. Subsequently a brick pier was found to be more stable and the wooden ones were discarded.
Of equal importance with the transit was the Chronometer. The expedition was supplied with four of these made by Negus of New York. They were regulated to sidereal time, and provided with a break circuit arrangement. This consists of a toothed wheel acting on a jewel pallet attached to a light steel spring. In this spring is a platinum point, which touches another platinum point, except when the spring is acted upon by the toothed wheel. These points are connected respectively with terminals on the outside of the chronometer, and are insulated from each other except at their point of contact. The electric circuit is complete through the chronometer except when the teeth of the wheel acting on the jewel pallet separate the points. The circuit is opened for about one-fortieth of a second and closed during the rest of the time. One tooth in the wheel is omitted and the circuit remains unbroken at that point which is the beginning of each minute. Each chronometer is provided with a condenser to take up the extra current, and avoid burning the contact points. These chronometers were most excellent instruments, the rate was generally small and very regular, and did not seem to be influenced in any way by the passage of the current. They are still in use, and are as efficient as ever.
The expedition was at first provided with a substitute for the chronograph in the shape of the old fashioned Morse telegraph register. In this a steel point or stylet was pressed by the action of an electro-magnet against a long fillet of paper, unwound by clock-work at a rate more or less regular. This magnet was in circuit with the chronometer and with a break circuit key in the observer's hand. As long as the electric circuit was closed the stylet made a continuous indented straight line on the paper; but as soon as it was broken, either by the chronometer or the observer's key, the stylet flew back and left the paper unmarked until the circuit was again closed. The effect of the action of the chronometer was to graduate the fillet of paper into a series of straight indentations, from one to two inches in length, separated by unmarked spaces from 1/16 to 1/8 inch in length. When the key was pressed an independent clear space was left on the paper, and by the relation in distance between the beginning of this space and the beginning of the second spaces immediately preceding and following, the time of pressing the key was determined. The omission of the break at the sixtieth second, made the mark of double length, and hence the beginning of the minute was easily recognized. These instruments served their purpose very well, but had several disadvantages. The rate of movement of the paper was not regular; when the clock-work was first wound up the motion was rapid and the second spaces long, and as the spring ran down the marks became shorter and shorter. Another drawback was the great length of the fillet; with spaces only an inch in length, it required five feet of paper to record a minute in time, and after a night's observation, there would be several hundred feet to examine, measure and record, occupying the greater part of the following day. By stopping the instrument between the observations something was gained in this respect, but this tended somewhat to confusion and error in keeping the record. They were only used for one season's work, and in their stead were procured two cylinder chronographs, made by Bond of Boston. These were fine instruments, but somewhat too delicate to stand the necessary transportation. In these instruments as in most other chronographs, a cylinder about six inches in diameter is made to revolve by clock-work once in a minute. An electro-magnet mounted on a carriage actuated by the same clock-work moves alongside the cylinder, in a direction parallel with its axis, at the rate of about an eighth of an inch in a minute. The armature of the magnet carries attached to it a pen, the point of which rests upon a sheet of the paper wrapped around the cylinder. While the circuit through the coils of the magnet is complete, the pen makes a continuous spiral line upon the paper, but when the circuit is broken by the chronometer, or key, it flies to one side making an offset, and immediately returns to its position, as soon as the circuit is again closed. The result is to graduate the whole surface of the paper into second spaces, from which the observations can be read off with the greatest ease.
For supplying the electric current, there was used at first, a modification of the Smee battery, but this proving very uncertain in strength, a gravity battery was substituted, and afterwards a number of LeClanché cells were procured.
Upon the first expedition, no telegraph instruments were carried, but the use of such as were needed was easily obtained from the telegraph companies. The line between Aspinwall and Panama was in good condition and no trouble was experienced in exchanging the time signals by which was effected the comparison of the chronometers. Wires were stretched from the observatories in each place to the respective telegraph offices, and for the exchange of signals were connected directly to the ends of the line.
Everything being ready, the routine of the work was as follows:—The transit being carefully leveled was placed in the meridian by observation of zenith and circumpolar stars. From six to ten time stars, and two or three circumpolars were then observed, the instrument was reversed in the Ys and nearly the same number of stars observed in the new position. At some time agreed upon, generally when the regular work of the telegraph line was over for the day, the wires were connected up and one of the operators came to the observatory to assist in holding communication. By a simple arrangement of relays, in the line and chronograph circuits the chronometer at one station was made to register its second beats on the chronograph at the other, which was all the time being graduated into second spaces by its own chronometer. This was done for about five minutes and the times of beginning and ending noted. Then the connections were reversed and both chronometers allowed to beat for five minutes on the chronograph at the first station.
This method of exchanging signals was only practicable on land lines or very short cables. The ordinary relay used on a land line requires a strong current to work it, and would not be affected in the least by the delicate impulse sent over a long cable, consequently when the expedition came to compare chronometers over the 600 miles of cable between Aspinwall and Kingston, it was necessary to use another method. At that time the instrument in general use on submarine cable lines was what is known as Thompson's mirror galvanometer. It consists of a coil of very fine insulated wire wound with great care on a spool or bobbin of vulcanite, about three inches in diameter and 1½ inches thick. In a hole in the centre of the spool is made to slide a small tube, so that the end of the tube will be in the centre of the coil. In the end of the tube is mounted a small mirror, swung in a vertical position on a single upright fibre of silk. Horizontally across the back of this mirror is secured a small permanent magnet, in length about the diameter of the mirror or about one-eighth to one-quarter of an inch. The mirror and magnet together weigh only one or two grains. When an electric current is sent through this coil it deflects the magnet and with it the mirror to the right or left. The apparatus is exceedingly sensitive so that it is influenced by very feeble currents. Communication has been maintained with an instrument of this kind over the Atlantic cables, by the current proceeding from a battery composed of a single copper percussion cap with a small scrap of zinc and a drop of acidulated water. The use of the mirror is to make visible the movements of the magnet. The coil is mounted upon a standard so as to be about eight inches above the table. At the distance of eighteen inches or two feet is placed a lamp. This is surrounded by a screen which cuts off all the light, except that which passes through a tube directed towards the mirror. Lenses in the tube focus the light on the mirror and thence it is reflected to a vertical white surface, a sheet of paper for instance, at a suitable distance and appears as a small and brilliant spot. A movement of the magnet causes a horizontal deflection of this spot to the right or left depending upon the direction of the current passing through the coil. As these movements can be produced at will by means of the key at the sending station, it is only necessary to apply to them the dots and dashes of the Morse alphabet, to have a very ready and perfect means of communication. To the uninitiated spectator the facility with which the practiced operator translates these apparently meaningless movements is remarkable. If the cable is long and not in good condition the signals are sometimes almost imperceptible, while any slight jar of the table or apparatus will produce a large and irregular effect. Earth currents also will cause vibrations hard to distinguish from the signals, and if, as sometimes happens, the battery is connected in the wrong way, the signals will be reversed. In spite of these drawbacks the skillful operator reads off the message and rarely makes an error. This instrument is still in use on some of the cable lines, but on most of them it has been replaced by a recording instrument, also the invention of Sir Wm. Thompson, which is almost as sensitive, and of which I will speak later on. The key used in connection with these instruments, both the mirror and recorder, is arranged with two levers, so connected that pressing one of them causes a current to be sent over the line in one direction, while the other sends it in the opposite.