Since the zero at Hull is the level of mean low water springs, this is also the reduction to M.L.W.S.

Example 2.-Find the correction to cast on July 14th at 8h 5, P.M., M.T.S. off Dingle.

Since the zero is 75 foot 9 inches below the level of M.L.W.S. the Reduction to M.L. W.S. is 8" to be subtracted from cast.

off Wicklow.

Example 3.-Find the Reduction on Sept. 27th at 4" 10m, P.M., M.T.S.

372 × 180

= 174°.

Since zero is '84 foot = 10 inches below the level of M. L.W.S.
Reduction to M.L.W.S. is 1 foot to be added to the cast.

This problem may also be utilised to find the depth of water to be found at any time at a given spot on a chart, by adding the Reduction found to the depth marked on the chart; if from this the ship's draught is subtracted,

the depth of water under the ship's bottom may be found; or, again, the time may be found when a vessel of given draught will find sufficient water to pass an obstruction.

EXERCISES.

Find the correction to be applied to the cast, in order to reduce the soundings to datum and M.L.W.S. :—

3.

4.

5.

6.

7.

8.

9.

99

1. On Oct. 14th, at 5h 56m P.M., M.T.G. off Hull.
2. July 24th, at 4h 30m A.M., M.T.S. off Galway.
Nov. 1st, at 1" 20 A.M., M.T G. off Grimsby.
Aug. 15th, at 1h 10m A.M., M.T.S. off Limerick.
Sept. 20th, at 6h 20m P.M., M.T.G. off Tenby.
Dec. 18th, at 10 P.M., M.T.G. off Spurn Point.
Aug. 6th, at 2h 10m A.M., M.T.S. off St. John (N.B.).
Nov. 3rd, at 7h 40m A. M., M.T.G. off Abu Shahr.
Sept. 28th, at 4h 45m A.M., M.T.G. off Fowey.
Oct. 19th, at 5h 20m P.M., M.T.S. off Exmouth.
Oct. 23rd, at 7h 10m P.M., M.T.S. off St. Mary's.
Oct. 7th, at 2h 40m P.M., M.T.G. off Beaumaris.
Sept. 13th, at 6h 20m A.M., M.T.S. off Workington.
Oct. 12th, at 0h 15m P.M., M.T.S. off Dartmouth.
Sept. 13th, at 5h 10m A.M., M.T.S. off Whitehaven.
Nov. 20th, at 5" 15" A.M., M.T.S. off Henjam Island.
July 15th at 10h 16 P.M., M.T.G. off St. Andrews (N.B.).
a Spring-tide day 13 hours from H.W. off Al Basra.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19. a Neap-tide day 2 hours from L.W. off Kuweit.

20. The depth of water on a bar at the entrance to a harbour at mean low water springs is 10 ft. 6 in. If the time of high water is 10" 51" A.M., find the depth of water on the bar at 9" A.M., the height of tide for day being 20 ft. 3 in. Spring range 22ft.

21. On a certain day it is high water at Falmouth at 7h 20m P.M. Height of tide 15 ft. 6 in.; mean tide level 8 ft. What is the earliest time in the afternoon when there will be not less than 20 feet of water over a shoal marked 2 fathoms on the chart?

22. At St. Helier, Jersey, the M.T.L. is 15 ft. 9 in., and a certain tide rises 25 ft. 6 in. Find the height of the tide 1h 30m before high water.

23. On Deal Bank the depth marked on the chart is 18 ft. What depth of water would there be over it on Feb. 6th at 1h 30m A.M. M.T.G.? If a vessel's draught was 25 ft., what depth of water would she have under her?

24. A vessel whose draught is 23 ft. 6 in. wishes to berth at inner harbour where average depth on chart is 2 fathoms. Heights of tide at high and low water on the day are 14′ 10′′ and 1′ 10′′; time of H.W. 9h 56m A.M., previous time L.W. 3h 43m A.M. What is the earliest time on the day when there

will be sufficient water?

CHAPTER IX.

TIME.

ART. 34. Time and Arc.-It is often necessary to change Time (ms) into Arc (°'"), or the contrary.

Twenty-four hours are the equivalent of 360°;

therefore 1h is equivalent to 15°

1m

15'

Two rules for converting Arc into Time are deduced from the above, viz.

NOTE. The latter is generally preferred, because of the easier division.

Find the equivalent values in Time of the following Arcs:

2° 59′ 30′′; 179° 0′ 45′′; 15° 0′ 15′′; 67° 52′ 55′′; 0° 17′ 20′′; 10° 19′ 40′′ 155° 11′ 36′′; 93° 0′ 45′′; 117° 38′ 30′′; 144° 54′; 176° 39′ 35′′; 3° 23′; 0° 59′ 45′′; 0° 0′ 15′′; 0° 15′; 0° 48′; 13°; 168° 9′; 61° 48′ 45′′.

By reversing the above rules, Time may be changed into Arc, viz.

(1) Multiply by 15.

(2) Multiply by 60 and divide by 4.