767.—Sounding Chains used for coast surveys are generally made of iron, but sometimes of brass. They are usually made of 10 fathoms entire length. The links are 1 inch, and the feet are indicated by tellers. The form of teller designed by the author is shown in Fig. 374 for the 3. A leaden weight, similar to that shown Fig. 375, is used upon the end of the chain—of 28 lbs., for ordinary coast work, or heavier if there are strong currents. The chain is contained in a strong wooden box.
Fig. 375.—Sounding line and weight.
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A very elaborate apparatus with steel wire line has been made for deep-sea sounding by Lord Kelvin and others; but this subject is beyond the province of the present work.
768.—Sounding Lines, used for survey of shallow coasts and harbours, are made of water-laid line of fine green hemp, about ¾ inch circumference, Fig. 375. White tapes are inserted as tellers at every foot, and red tapes at every fathom. 3 to 6 fathoms are the ordinary lengths employed. If the water is shallow the fathoms are easily counted, but if thought necessary knots may be tied to indicate the number of fathoms on the red tellers. The weight is about 7 lbs. for 50 feet line, about 15 lbs. for 100 feet. The under side of the weight is commonly recessed to take tallow when it is desired to bring up a specimen of the bottom, if this is loose sand or mud.
769.—Coast Survey Lines.—For surveying distances, from point to point of soundings along a coast, lines of fine copper wire rope marked with tellers at 50 and 100 feet are commonly used. The line is generally allowed to rest on the bottom of shallow water, and is floated up by means of attached corks in deep water. It is usually laid and picked up by means of a reel fixed at the stern of the surveying boat. The lengths of line used vary from 1000 to 5000 feet.
770.—Telemeters.—These scarcely enter within the practical limits of surveying instruments, but as several attempts have been made to introduce their use it is necessary to mention them. The general attempt has been to measure a great distance, 1000 feet or more, by means of the angles subtended from the ends of a short base to a distant point. This base in the telemeter of Piazzi Smyth is 60 inches; Colonel Clarke, 72 inches; Otto Struve, 73·5 inches; and Adie, 36 inches. The angles are usually taken upon the principle of the sextant by coincidence of image. Very much greater success has been attained recently by Messrs. Barr and Stroud by means of their range-finder of 54 inches base. The author, as far as his information reaches, is assured that no instrument of the class is satisfactory for surveying purposes. Further, the subject is one to which he has devoted some study, and designed two telemeters.[55] One of these appeared to him for a time satisfactory within certain limits. The base in this instrument was 50 feet, formed of a fine pianoforte wire stretched between two observing telescopes, the tension of the wire directing the one telescope to a right angle, and the other telescope to an arc which read either degrees and minutes or absolute distances in the eye-piece to the direction in which the telescope was pointed. In first trials this instrument was found fairly satisfactory; but subsequently in windy weather the deflection of the wire rendered the action of a pair of instruments quite unreliable.
There are some instruments, as Colonel Gautier's telemeter used in the French army, which depend upon combined reflectors placed normally at 15° to 45°, as in the apomecometer, [art. 693], but with a tangent screw to give a small motion of displacement to one mirror which reads on a scale of calculated distances to angle from a certain base measured between two stations of observation. A very similar instrument, invented by Labez, has one reflector only at 45°. These instruments may be useful for measuring approximate distances for range in the army, but can scarcely rank as surveying instruments, the box sextant, [art. 664], being in every way a superior telemeter for the purpose when a measured base can be fixed and well-known trigonometrical calculation used.
771.—The simplest and best telemeter for surveying purposes is the subtense telescope, and all good, up-to-date surveying instruments have their telescopes so fitted, but for those who do not carry an instrument with a telescope the reviser has designed a small subtense telemeter, Fig. 376, which consists of a small telescope fitted with subtense points, and mounted in a collar which has vertical and horizontal motions and a centre socket to fit a Jacob's staff. The stadia is set to read 1 in 100. The telescope has rack and pinion focussing and may be revolved in its socket so that the stadia rod may be read held either horizontally or vertically. It is packed in a leather holster case, and a four-fold 10-feet spring-pointed stadia rod is supplied with it divided into feet, tenths, and hundredths.