Part 11 (1/2)

AD sin CAD 186615 sin 59 20'

or CD = ---------- = ------------------- sin SCD sin 85 9' 53”

or log CD = log 186615 + L sin 59 20' - L sin 85 9' 53”

= 32709456 + 99345738 - 99984516

= 32070678 'CD = 161090 ft

The distance between any two positions of the float can be obtained by calculation in a sith C D was obtained, but this is a lengthy process, and is not necessary in practical work It is desirable, of course, that the positions of all the stations be fixed with the greatest accuracy and plotted on the map, then the position of the float can be located with sufficient correctness, if the lines of sight obtained froles read with the theodolites are plotted, and their point of intersection marked on the plan The distance between any two positions of the float can be scaled from the plan

The reason why close measurement is unnecessary in connection with the positions of the float is that it represents a single point, whereas the sewage escaping with considerable velocity from the outfall sewer spreads itself over a wide expanse of sea in front of the outlet, and thus has a tangible area The velocity of any current is greatest in the centre, and reduces as the distance froes of the current are lost in comparative still water; so that observations taken of the course of one particle, such as the float represents, only approxih the sea Another point to bear in reat that it is generally only by reason of the unbroken faecal, or other matter, that it can be traced for any considerable distance beyond the outfall It is unlikely that such matters would reach the outlet, except in a very finely divided state, when they would be rapidly acted upon by the sea water, which is a strong oxidising agent

CHAPTER XV

HYDROGRAPHICAL SURVEYING

Hydrographical surveying is that branch of surveying which deals with the complete preparation of charts, the survey of coast lines, currents, soundings, etc, and it is applied in connection with the sewerage of sea coast tohen it is necessary to determine the course of the currents, or a float, by observations taken from a boat to fixed points on shore, the boat closely following the float It has already been pointed out that it is preferable to take the observations from the shore rather than the boat, but circumstances may arise which render it necessary to adopt the latter course

In the si the cos of two known objects on shore For exa 37 may represent the positions of two prominent objects whose position is hbourhood, or they staffs specially set up and noted on the s of A and B are taken by a prisnetic variation be N 15 W, and the observed bearings A 290, B 320, then the position stands as in Fig 38, or, correcting for39, fro of C from A will be 275-180=95 East of North, or 5 below the horizontal, and the true bearing of C from B will be 305-180=120 East of North, or 35 below the horizontal

These directions being plotted will give the position of C by their intersection Fig 40 shows the prismatic compass in plan and section It consists practically of an ordinary coht-hole at one side, and a corresponding sight-vane on the opposite side When being used it is held horizontally in the left hand with the prisht-vane raised When looking through the sight-hole the face of the compass-card can be seen by reflection from the back of the prism, and at the sahted with the wire in the opposite sight vane, so that the bearing of the line between the boat and the required point may be read If necessary, the co the stop at the base of the sight vane In recording the bearings allowance netic variation for the year 1910 was about l5 1/2 West of North, and it isnearer to true North at the rate of about seven minutes per annum

[Illustration: FIG 37--POSITION OF BOAT FOUND BY COMPass BEARINGS]

[Illustration: FIG 38--REDUCTION OF BEARINGS TO MAGNETIC NORTH]

[Illustration: FIG 39--REDUCTION OF BEARINGS TO TRUE NORTH]

There are three of Euclid's propositions that bear very closely upon the proble object with regard to the coast, by observations taken froles of every triangle are together equal to two right angles”; Euclid III (20),