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W LOW WATER NEAP
с SPRING TIDES
VERTICAL SECTION OF BOTTOM OF THE SEA
EXPLANATION OF THE DIAGRAM.
a d is the Sounding by cast of the Lead. cd Sounding by Chart. a b quantity found in Table B, Admiralty Tables. b c i mean Spring Range.
quantity to be taken from the cast to reduce it to
the Chart Soundings.
The Soundings on Charts are laid down for the depth at low water of spring tides.
TO FIND THE DEPTH OF WATER AT ANY
TIME OF TIDE.
RULE 1.–Find the time from high water.
2. Find the difference between the height of the tide and the } mean spring range, as given under the month and place in the Admiralty Tide Table.
3. Enter Table B with the above differences; the quantity there found is to be applied to the } mean spring range.
4. The result is the amount to be subtracted from the cast to give the soundings in the Chart.
EXAMPLE 1.-1866,* July 26th. At 4h. 10m. A. M.,
off North Shields, the depth of water by the lead was 9 fathoms. Required the Sounding on the Chart.
* The Examples for the above are given for the year 1866, as the Admiralty Tide Tables for 1870 could not be obtained.
ft. in. Time of H.w., Admiralty T.T. 2 25 Height of tide 11 4 Time of cast.
.4 10 } mean spring rạnge 6 8
EXAMPLE 2.-1866, December 9th, at lh. 5m. P. M.,
in coming up the Firth of Forth, the lead shewed 12 fathoms of water. What was the sounding on the chart.
ft. in. Time of 1.w. by Tables 3 36 Height of tide .......15 2 Time of cast. .1 5 i mean spring range
1.—1866, March 27th, at 3h. 30m. A. M., in the Bristol Channel, the sounding by the lead was 34 fathoms. Find the depth by chart ?
2.—1866, September 2nd, at 7h. 15m. P. M., going up the Swin, the depth by the lead was 9 fathoms. Find the sounding as marked on the chart ?
3.-1866, January 10th, at noon, coming up the Channel in thick weather, by the reckoning are off the Owers, on taking a cast of the lead we find 16 fathoms. What depth must I look for on the chart ?
The Log Line is used to denote what distance a ship runs, and the length of a knot on it must bear the same proportion to a nautical mile as the number of seconds in which the glass runs does to the number of seconds in an hour.
The practical rule for finding the number of feet and inches is—Add a cipher to the number of seconds in which the glass runs, and reckoning this as feet, divide by 6; the remainder multiplied by 2 will give the inches.
28 glass gives 46
32 glass gives 53
8 0 4
The Lead Line has nine marks and eleven deeps, marked as follows:
At 2 fathoms, leather.
leather, with a round hole in it.
a piece of cord, with 2 knots.
white rag: red rag:
blue rag white rag. red rag
The Deep-sea Lead Line is marked in the same way as far as 20 fathoms, after which a knot is placed for every additional 10 fathoms; that is,
30 fathoms, 3 knots.
4 knots. and so on. Every 5 fathoms is marked either with a single knot, or (which is preferable) a piece of leather.
HINTS TO CAPTAINS AND MATES ON THE USE OF THE BAROMETER AND THERMOMETER.
While in some localities the pressure of the atmosphere” has been known to equal the weight of a column of pụre mercury nearly 31 inches in height, in others (at the sea-level) it has balanced only 27} inches. Thus we see that the atmosphere possesses in itself such varieties of pressure, as to render it impossible to remain at rest. The exchange of masses of air, so necessitated, is constantly going on, either gently by means of moderate and fresh breezes, or violently through the agency of storms. Originally generated by the Sun's gradual action, (probably aided by less understood causes, lunar and magnetic,) atmospheric changes are accomplished gradually, and hence may be "predicted,” if proper precautions are taken.
To predict weather-changes, two means must be employed.
1st,-Instruments, such as the Barometer, Thermo
2nd,-Appearance of the sky, clouds, &c. The two should be compared with each other, and neither should on any account be neglected.
THE BAROMETER AND ITS USES.
A Barometer is simply a machine for weighing the pressure of the atmosphere. This it does by balancing a column of pure mercury in an upright glass tube.
The tube is quite open at the lower end, where it stands in a cistern partly filled with mercury, to which the atmospheric pressure has free access. The upper part of the tube, above the mercury, is emptied of air and effectually closed. Thus all the pressure is at the lower or cistern end. Hence, when the atmosphere is heavy the mercury will be pressed up the tube, or "stand high ;” and, contrarily, when the atmosphere becomes less weighty, the mercury will “fall."
If, therefore, the Barometer is rising, we infer that the atmospheric pressure is increasing, and if falling, thạt the pressure is lessening. Further, if the glass rises or falls slowly, the change thus occurring with deliberation