Chap. VIII.
8-19
Subterranean Surveying is performed with the miner’s compass, which for observationsof horizontal angles in by no means sufficiently accurate. The circumferentor or halftheodolite is more so, and may be used with greater certainty, though the common theodoliteshould always be preferred : it is often of more importance to have an accurate plan of theworks in a mine, than of the land above it. The surveys of pits or mines being not onlymade to direct the working, but for the sinking of shafts, ventilation, or for the raisingthe produce, after the situation of the shafts are determined on above ground, the work-ings below must be either set out on the surface, or the survey must take place beneath.To adapt the theodolite to mining observations, the stand on which it rests should be soformed, that it can be readily disengaged.
The survey is commenced with the assistance of two men, who carry the chain, and twoothers the lamps, one of which is placed at the starting point from whence the measure-ment is to commence, and the theodolite is advanced in the direction of the line, as far asit is possible to obtain a sight of the lamp at the point advanced from. 'Die vernier of thetheodolite being fixed at zero, the telescope is directed towards the light, and the angle ofinclination noted down, another lamp is advanced along the line and the same observationstaken : the distances are then carefully measured from one station to the other, as practisedin roadway surveying ; all underground surveys should be made to agree with those takenon the surface, which may be readily done by comparing the adits and shafts, both as totheir distances and bearings to each other.
Care must be taken to notice whether the needle is affected when the theodolite is used,which is sometimes the case; when the instrument is elevated 2 or 3 feet above thetram plates of a railway, there does not seem to be any sensible attraction between themand the needle.
Maritime Surveying , comprises the laying down charts with accurate representations ofthe coasts and harbours, and is one of the most important applications of the science, as it en-ables the mariner to pursue his voyage and return to his port without encountering thosedangers which beset him in the shape of rocks and shoals, shallows and flats. The firstthing to be performed for the construction of a map or chart is to refer to some fixed pointson the shore of the coast to be laid down ; these are ascertained with great precision bymeans of a trigometrical survey on shore.
Tide Gauges arc erected in well-chosen localities in a vertical position, and divided intofeet and tenths : the zero point of each gauge corresponds with a bench mark permanentlyfixed, so that should any he displaced by the violence of the sea, they can, by means of aspirit level, be refitted in their original position.
These gauges serve after a series of observations to give the lowest point of the lowesttide at full and change of the moon, and to the level of this lowest point the depths of allthe soundings are referred : they serve also to show the rise and fall of the tide, by whichmeans all the registered soundings are reduced to the lowest level: this is very essential,as it is not practicable to take all the soundings over an extensive bay at the precise time oflow water : near the shore these observations may be made with reference to some permanentmarks made on the walls of the quays.
A second gauge is placed further out at sea, so that when the tide has left the first, thesecond may be observed. Then the zero division of the first gauge must be compared, bymeans of the spirit level, with some division of the second ; and this, as well as each succes-sive gauge, must be denoted by some number or letter, and entries made to record the timeof changing from one gauge to another. A situation should be selected where the base ofthe tide gauge is not left dry at low water.
When an observation is to be made, a person with a well-regulated watch is stationed ateach tide gauge for the purpose of registering the height at every quarter of an hour orother stated intervals. A meridian line is marked upon each station, so that the observermay regulate his watch by the course of the sun.
The time of high water at the full and change of the moon should he carefully markedon the tide gauge, and this time may be either mean or apparent, but whichever is selec-ted must be noted; the instant to be registered is that when the surface of the water isthe highest, hut if the water is perfectly calm, its change when near the highest point isslow, and it almost seems stationary for some moments. To prevent the effect of wavesrendering the surface uncertain, an upright pipe is sometimes fixed by the side of the gaugein such a situation that at low tide the water only reaches the lower part: the bottom ofthe pipe must be stopped, and a number of small holes drilled in it near the bottom abouta quarter of an inch in diameter. A float is then placed on the pipe which carries a lightrod, and by means of feet and inches marked on it, the rise of the tide can easily be read off.
In narrow channels or at the mouths of rivers, a great many gauges are requisite, and noprecise number can be mentioned, as that must depend altogether upon circumstances.Churches, lighthouses, are then made use of on land, to serve as vertices to the trianglesabout to be laid down, and it is usual to place signals upon them; and those which serve tobe viewed from the sea are painted white, when the ground falls behind them, but of a
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