Page:The American Cyclopædia (1879) Volume IV.djvu/776

 760 COAST SURVEY ployed. The manner in which this experiment is made is essentially as follows : An astrono- mical clock is so connected with the telegraph wires as to graduate by its beat slips of paper delivered with uniform velocity into spaces rep- resenting seconds of time, each about an inch in length, both at the place where the clock is and at some distant station. If now, at the latter station, arbitrary signals are made be- tween those given by the clock, and transmit- ted to the clock station, the corresponding marks on the register of that place will appear later, or more distant from the preceding clock marks than on the register where they are made. The difference measures the whole time of transmission from the former to the latter station and back again. On the Pacific coast, besides the telegraphic determinations already spoken of, longitudes have been determined by the astronomical methods and also by chrono- metric expeditions. In 1860 an expedition was undertaken from Point Hudson, Admiralty in- let, to Gray's harbor, Washington territory. In 1861 another was made between San Fran- cisco and Coos bay in Oregon. In 1867 the first Alaska expedition was undertaken from San Francisco, during which the ports of Vic- toria, Port Simpson, Sitka. St. Paul's harbor, Captain's harbor, Port Lincoln, and Stony island were visited. In 1869 the second Alaska expedition took place, and Astor point in Columbia river, Esquimalt on Vancouver's island, Victoria, Fort Wrangel, Kok-klux, and Sitka were visited. In 1869 an expedition was made from San Francisco to Eureka in Hum- bolt bay, California; and in 1870 another between San Francisco and Punta Arenas. Azimuth. The direction of the meridian is determined by observations on the pole star, or other close circumpolar stars, at the time of their passing the meridian, or of their greatest eastern or western elongation. The angle be- tween the vertical plane passing through the star and that passing through one of the trian- gle sides is measured with a theodolite. The angle which the former plane makes with the meridian can be computed from the time of observation and the star's place ; and thus the azimuth, or true bearing of the triangle side, becomes known, as well as the bearing of all other sides connected with it by triangulation. Topography. This is a representation on paper of the natural features of the country. The outlines of the shore, the irregularities of the surface, the forms and dimensions of hills, forests, streams, rocks, meadows, towns, and villages, are all represented by certain conven- tional modes of drawing, well understood by those who make use of the maps. The draw- ing presents to us the surface of the earth as it would appear to an observer from above it. The surveyor uses a table or board on which is fitted a sheet of drawing paper. On the latter several points of the triangulation are already plotted. For the mapping and delineation of intermediate points use is made of the alhidade in connection with the plane table, as the table mentioned above is called. The plane table is a well seasoned drawing board, about 30 inches in length and 24 in width ; it is usually com- posed of several pieces of wood grooved to- gether in such a manner as to prevent warping, and is supported upon three brass arms which are connected with and supported by a tripod arranged with levelling screws. The sheet upon which the country is to be mapped is fastened to the table by means of brass clamps. The alhidade consists of a brass rule about 22 inches long, having a circular level on its upper face. Near the middle of the rule is a perpen- dicular cylindrical column of brass called the standard ; on the top of this standard is a square brass plate, which supports four columns ; these in their turn uphold two cross pieces, upon which rests the axis of a telescope. To one end of the axis is attached a vertical arc, which is used in the measurement of vertical angles for heights. The telescope itself has the usrual cross bars and means of focal adjustment. In practice the operation of the plane table is as follows : The surveyor places his table upon the ground, properly adjusts it by the levelling screws, and determines its position on the sheet by drawing lines on the paper, in the directions of at least three of the trigonometrical points. The intersection of these lines on the paper will give the position which his table ought to occupy on the map. Having fixed this point, he directs the telescope of his alhidade to any prominent points in sight, and draws lines in their direction by means of the ruler. Then going to a second point, and fixing his position in the same way, he sights to the same objects as before from his new position. The lines thus drawn on the sheet will fix by their inter- section the places of all these points on the map. It is in fact a practical continuation of the triangulation on a minute scale. Acci- dental details of ground and slight irregulari- ties of shore line may be drawn by the eye. The topographical maps are generally surveyed on a scale of -1-5,^5^ of the natural dimensions. In localities where a great amount of detail is to be represented, such as large cities and their vicinity, a scale of s % 66 is employed ; and in some special surveys so large a scale as j^ has been used. On the other hand, on flat and thinly settled ranges of the coast, a scale of TTsytanr i 8 employed. The extent of ground represented upon a single topographical sheet depends upon the scale. On a scale of -ft.hnH or about six inches to the mile, a square foot of the drawing represents about four square miles of the surface of the earth. The metre chain, which was frequently used in topo- graphical work to determine distances where the usual means of intersection could not be applied (as in roads, on beaches, &c.), is now to a great extent superseded by a simple but very useful instrument called a telemeter. This, as used in the coast survey, is simply a scale of equal parts, painted upon a wooden rod