opposite to each hour, from low water is shown the height which the level of the water would mark upon a staff the 0 of which was at low water.

TABLE A.

Showing the rate of rise and fall of the tide at New York and Liverpool.

Hours before

or after Low water.

After.

Before.

Height of tide.

New York. Liverpool.

hrs.

ft.

6

4.2

ft. 18.9

3.7

16.2

2.9

10.4

1.8

6.2

0.9

3.0

This curve or this table will enable the navigator to conjecture the probable rise and fall from low or high water at ports where the rise and fall is about the same as at New York or at Liverpool, but will not apply to others.

If watching this tide staff from day to day in some port upon our coast we should note the time of high and low water, and the height beginning with, say, two days after change day of the moon, and continuing for a lunar month or twenty-eight days, we should find that on that day the lunitidal interval was nearly the average of all which we would obtain in the course of the month, and that the water rose higher and fell lower than at any other high and low water. These are spring tides. The interval would go on decreasing until two days before the first quarter, when it would reach its least value. The height of high water would decrease, and of low water increase, until one day after the first quarter, when the one would reach its least and the other its greatest height, corresponding to neap tides, or least rise and fall of the water. From the period of its least value to three days before the full the lunitidal interval would increase and then decrease, and so onward to two days after the full, when the interval would have its average value again, and the heights would again correspond to spring tides. The corresponding changes in the lunitidal intervals and heights take place from the full to change, passing through the moon's third quarter. It is hardly necessary to remind the navigator that at change the moon and sun cross the meridian together, or the hour of transit is 0 hrs., and that at the first quarter the hour of transit (moon's southing) is 6 hrs., at the full, 12 hrs. This change in the lunitidal interval runs its course from change to full or full to change, that is, in a half lunar month; it is hence called the half monthly inequality, and is in general the largest of the changes in the lunitidal interval, which must be taken into account.

If there were no changes in the lunitidal interval, it would be very simple to determine the time of high or low water at a place. A table of intervals and an almanac showing the time of transit, or as it is sometimes called in the almanacs the time of the moon's southing, would be all that is necessary. Suppose we wish to determine the time of high water at Boston on the 12th of December, 1859. From the table of establishments, No. LV., we take that of Boston, 11h. 27m.; from a Boston almanac, the time of the moon's upper transit on that day 1h. 59m., A. M., adding the two numbers we have 13h. 26m., or 1h. 26m., P. M., as the time of high water. The corresponding low water is 6h. after, or more exactly 6h. 13m. So, if the heights did not change, one number in the table would give us the rise and fall. This supposes that we had an almanac of the port at which we desired to know the time of high water, but as this would usually not be the case, we must take our result from the Nautical Almanac, with which we are provided. This referring to the time of transit of the moon over the meridian of Greenwich, and to the same meridian for the longitude, 2m. must be added to the time of transit at Greenwich for every hour of west longitude, and subtracted for every hour of east longitude. The same result may be had from the table B, where the numbers to be added to the time of the moon's transit are given for every ten degrees of longitude.

RULE I.- Find the time of the moon's coming to the meridian of Greenwich, on the given day in the Nautical Almanac. Enter Table B and find the longitude of the given place in the left hand column, corresponding to which is a number of minutes to be ap plied to the time of passing the meridian at Greenwich, by adding when in west longitude, but subtracting when in east longitude; the sum or difference will be nearly the time that the moon passes the meridian of the given place.

To this corrected time add the time of high water or full sea from Table LV. The sum will be the time of high water on that day.

EXAMPLE I.-Required the time of high water at Charleston (S. C.), November 19, 1859, in the afternoon, civil account. From the Nautical Almanac we find the moon's meridian passage at Greenwich, November 18, at 19h. 26m., which corresponds to 7h 16

30

40

50

60

70

9

80

11

90

12

100

14

110

15

120

16

130

18

26m., A. M., of the 19th day by civil account. From Table LIV. we have the longitude of Charleston 79° 54′ W., which, for this purpose, may be assumed as 80°. Entering Table B with 80°, we find the correction of the moon's passing the meridian to be 11 minutes, which is to be added as the longitude is west. The moon's meridian passage at Charleston is therefore at 7h. 37m., A. M. Adding to this the luniti ial irterval 7h. 13m. from Table LV. we obtain 14h. 50m, or 2h. 50m., P. M., as the time of high water at Charleston in the afternoon of November 19, 1859.

་

EXAMPLE II.-Required the time of high water at Portland (Maine), December 13, 1859, in the afternoon, civil account. The Nautical Almanac gives the moon's meridian passage at 14h. 47m. on the 12th, corresponding to 2h. 47m, A. M., on the 13th. The longitude of Portland is 70° 12′ W., in time (Table XXI.) 4h. 41m. At the rate of two minutes for every hour of west longitude we should add 9m. to the Greenwich time of the moon's meridian passage, giving it for Portland at 2h. 56m. Adding the lunitidal interval from Table LV. 11h. 25m., gives 14h. 21m., or 2h. 21m., P. M., for the time of high water on December 13th.

These results would be the time of high water, did not the lunitidal interval vary.

If the changes of lunitidal interval from half monthly inequality were the same for all ports, it would be easy by a table of a single column to apply the required correction to the time of high water when the moon was not at full or change, but this is not the case. It has been found, however, that the general law of this change is the same, and that by knowing the greatest and least lunitidal interval for any port we can determine by computation the change of interval. The ports having nearly the same difference of greatest and least interval are grouped together, and the correction to be applied to the establishment, according to the age of the moon, is given in Table C. The ports which may thus be classed together are the following: a. The ports of England and of the western coast of Europe in general. b. The ports on the eastern or Atlantic coast of the United States. c. The ports of the western coast of Florida and of the western or Pacific coast of the United States.

This table is arranged on the supposition that the corrected establishment is used, which is the case for the more important ports in Table LV.

ple I., given before, the time of the moon's meridian passage being 7h. 37m., we enter the table with that quantity in the column of time of the moon's transit, and under the head of group b, and by an easy proportion we find the correction to the lunitidal interval to be, "add 3," that is, three minutes must be added to the mean lunitidal interval at Charleston, making it 7h. 16m., which, added to the time of moon's transit, would give 2h. 53m., P. M., as a more accurate time for the high water of November 19, 1859.

In Example II. we had the time of the moon's transit at Portland at 2h. 56m., entering Table C with 3h. in the column of moon's transit (which is near enough for this purpose), we find in the column of group b a correction of "subt. 16m.," i. e., sixteen

which added to the time of moon's transit, gives 14h. 5m., or 2h. 5m., P. M., for the time of high water on December 13th.

The changes of the moon in declination cause a tide once in twenty-four lunar hours, which adds itself to the morning high water, increasing it, and subtracts itself from the next, or afternoon, high water, or vice versa This is called the diurnal inequality. It affects the time and the height of both high and low water. In most of the ports of the Gulf of Mexico this diurnal tide is the only marked one, except when the moon is near the equator. In the ports of Great Britain and Ireland, France and Spain, the diurnal inequality in height is marked, but in time is inconsiderable. On the Atlantic coast of the United States it is small both in time and height. It increases in passing along the straits of Florida to the western coast of the Florida peninsula, and the semidiurnal tides almost disappear from Cape San Blas to the mouths of the Mississippi, reappearing only slightly between Isle Dernière and Galveston, and again being merged in the diurnal tide from Aransas Pass to Vera Cruz, and probably southward. The small tide of the day is frequently called by navigators a half tide, and in speaking of the large and small tides of the day they say the tide and half tide. On the western coast of the United States this inequality is large both in time and height, amounting at San Francisco at its greatest value to two and a half hours of time and four feet of height. It is probably large on the whole western coast of South America, but observations are wanting to give information in regard to the tides of these localities.

The following table will give the corrections for the daily inequality in time and height for the Facific coast of the United States to within about eight minutes of time and and three inches of height.

The quantities in this table are the corrections to be applied to the times of high or low water obtained by means of Rule I and corrected by Table C.

RULE.-Find from the Nautical Almanac the number of days elapsed since the moon's declination was greatest, or if before, the number of days to come to that time. With this enter Table D in the first column, and opposite the number find the correction in the second colum. When the moon's declination is north, the correction is to be subtracted; when south, it is to be added. When the moon's declination is nothing, the correction is nothing. The fourth and fifth columns give the corrections to the heights of mean high water and mean low water for the same

days. The corrections for the height of low water follow the same rule as those for the times of high water; but for the heights of high water they are the contrary, that is, they are to be subtracted when the former are to be added, and vice versa.

The effects of this inequality may be also expressed in the following way: The moon's declination being north, the high water next following the moon's transit will be earlier and higher than the average, the next low water later and lower, the next high water later and lower, and the next low water earlier and higher; when the moon's declination is south, the first high water is later and lower, the next low water earlier and higher, the next high water earlier and higher, and the next low water later and lower, by the amounts given in the table.

EXAMPLE. Required the time of high water at San Francisco, October 16, 1859. By Rule I., we find the moon's transit to happen at 3h. 21m., A. M., on that day. The establishment for San Francisco, from Table LV., is 12h. 6m., which added to 3h. 21m., gives 15h. 27m., or 3h. 27m., P. M., as the time of high water, uncorrected for the half monthly and diurnal inequalities. The former is obtained from Table C, group c, and is 45m., which is to be subtracted, giving 2h. 42m.; the second is obtained from Table D. By referring to the Nautical Almanac, we find that on the given day the moon had her greatest declination north. Entering, therefore, the table with 0 day from greatest declination, we find corresponding to it in the second column 64m., to be subtracted, as the declination is north, giving 1h. 38m. as the time of high water. If the corrections had been neglected, we should have been nearly two hours in error. The same table tells us in the other columns that this high water would be 1.0 foot higher than an average high water, and the next low water 1.8 foot lower. The next high water, A. M., of the 17th, would be one foot lower than the average, or two feet lower than the above high water, the next low water 1.8 feet higher than the average, or 3.6 feet higher than the preceding one.

1

CURRENTS.

A CURRENT is a progressive motion of the water, causing all floating bodies to move that way towards which the stream is directed. The set of a current is that point of the compass towards which the waters run, and its drift is the rate it runs per hour. The most usual way of discovering the set and drift of an unknown current, is the following, supposing the current at the surface to be much more powerful than at a great distance below the surface:

Take a boat a short distance from the ship, and, by a rope fastened to the boat's stern, lower down a heavy iron pot or loaded kettle to the depth of 80 or 100 fathoms; then heave the log, and the number of knots run out in half a minute will be the miles the current sets per hour, and the bearing of the log will show the set of it. There is a very remarkable current, called the Gulf Stream, which sets in an north-east direction along the coast of America, from Cape Florida towards the Isle of Sables, at unequal distances from the land, being about 75 miles from the shore of the southern States, but more distant from the shore of the northern States. The width of the stream is about 40 or 50 miles, widening towards the north.

We were first indebted to Doctor Franklin, Commodore Truxton, and Mr. Jonathan Williams, for the knowledge we possess of the direction and velocity of this stream. Its general course, as given by them, is marked on the chart affixed to this work. They all concur in recommending the use of the thermometer, as the best means of discovering when in, or near, the stream; for it appears, by their observations, that the water is warmer than the air when in the stream; and that at leaving it, and approaching towards the land, the water will be found six or eight degrees colder than in the stream, and six or eight degrees colder still when on soundings. Vessels coming from Europe to America, by the northern passage, should keep a little to the northward of the stream, where they may probably be assisted by a counter current. When bound from any southern port in the United States of America to Europe, a ship may generally shorten her passage by keeping in the Gulf Stream. By steering N. W. you will generally cross it in the shortest time, as its direction is nearly N. E. (See page 6, Notes and Corrections.)

In other parts of the Atlantic Ocean, the currents are variable, but are generally south-easterly along the coast of Spain, Portugal, and Africa, from the Bay of Biscay towards Madeira and the Cape de Verds. Between the tropics, there is generally a current setting to the westward.

There is also a remarkable current which sets through the Mozambique Channel, between the Island of Madagascar and the main continent of Africa, in a southwesterly direction. In proceeding towards Cape Lagullas, the current takes a more westerly course, and then trends round the cape towards St. Helena. Ships bound to the westward from India, may generally shorten their passage by taking advantage of this current. On the contrary, when bound to the eastward, round the Cape of Good Hope, they ought to keep far to the southward of it. However, there appears to be a great difference in the velocity of this current at different times; for some ships have been off this cape several days endeavoring to get to the westward, and have found no current; others have experienced it setting constantly to the westward, during their passage from the cape towards St. Helena, Ascension, and the West India Islands. Instances have however occurred, where an easterly current was experienced off the Cape of Good Hope. Off Cape Horn there is a current setting N. 80° E., at the rate of 12 miles the 24 hours, during the summer months-during the autumn months it is accelerated nearly double, and sets N. 49 E.

The following is compiled from a communication of Lieut. Bent to Mr. G. W. Blunt, respecting a stream of warm water, which is found on the east coasts of Formosa and the Japan Islands.

This stream has its origin in the great Equatorial current of the Pacific, from which it is separated by the south end of Formosa, whence it is deflected to the northward along the east coast of that island, until reaching the parallel of 26° north, when it bears off to the northward and eastward, washing the whole southeast coast of Japan as far as the Straits of Sanger.

tween the islands of Formosa and Majico-Suica, with a breadth of 100 miles; but to the northward of the latter it expands rapidly on its southern limit and reaches the Lew-Chew and Bonin groups, attaining a width to the northward of the latter of 500 miles. The north-western edge of the stream is strongly marked by a sudden change in the temperature of from 10° to 20°; but the south-eastern limit is less distinctly defined. Along the borders of the stream, and also in its midst, where whirls and eddies are produced by islands and inequalities in its bed, strong tide rips are encountered.

The average strength of the current between the south end of Formosa and the Straits of Sanger is from 35 to 40 miles per day. Its maximum once off the Gulf of Yedo was observed as high as 72, 74 and 80 miles respectively per day. A cold counter-current may exist to the north of 40° and long. 143°, running through the Straits of Sanger; but to the westward of a line connecting the north end of Formosa and the south-western extremity of Japan, a cold current sets to the southward, through the Formosa Channel, into the China Sea.

This current is well known to the navigators trading on the coast of China, who never, in the north-east monsoon, attempt to beat against it, but make the passage usually to the eastward of Formosa.

The Japanese call this warm stream, setting along their southern shores to the northward and eastward, the Kuro-Sicoo, or Black Stream, from its deep blue color Its maximum temperature is about 86°, and the difference between its temperature and that of the ocean due to the latitude is on an average about 12°.

There is no counter-current intervening between the Kuro-Sicoo and the coast of Japan south of the Straits of Sanger, consequently the large body of warm water which washes the shores of the island must essentially contribute in modifying its climate.

All cases of sailing in a current are calculated upon the principle that the ship is affected by it in the same manner as if she had sailed in still water, with an additional course and distance exactly equal to its set and drift. On this principle the projection and calculation of any problem of this kind may be easily made

EXAMPLE.

If a ship sail 98 miles N. E. by N., in a current which sets S. by W. 27 miles, in the same time, required her true course and distance.

BY PROJECTION.

Describe the compass NESW; through the centre A draw the N. E. by N. line AC equal to 98 miles; through C draw the line BC parallel to the S. by W. line, make BC equal to 27 miles, and join AB. Then AB will be the course and distance made good; and by measuring, we find the course to be N. E. & N., the distance 74 miles.

N

NbE

NEIN 98

b SbW 27