PARALLAX.

As we observe in navigation the central altitude of bodies (as in the sun when we allow for semi-diameter), so it is that we should observe them from the center of our body, instead of from the earth's surface. Not being able to do so, we can allow for it by the use of Table XV; but, as the parallax of the sun never exceeds 9", it will be found profitable to ignore it entirely, when considering practical navigation. Parallax always makes an altitude lower than it should be; consequently, when the correction is used, it must always be added to the observed altitude.

REFRACTION.

Before the rays of light from heavenly bodies can reach an observer's eye upon the surface of the earth, they must pass through the earth's atmosphere; and this quantity becomes denser the nearer the rays of light approach the earth; hence the path of a ray of light becomes a curve by passing through mediums of varying densities. The difference in the direction of a ray of light, before and after entering the atmosphere, is what is called refraction.

Refraction varies with the temperature and density of the air, increasing by cold and greater density, and decreasing by heat and rarity of the atmosphere. Owing to refraction, objects appear higher than they really are, except when in the zenith, where there is no refraction; but the less the altitude, the more obliquely the rays will enter the atmosphere, and the greater will be the refraction. (See Table XX.) At the horizon the refraction is the greatest. In consequence of refraction, any heavenly body may actually be below the horizon when appearing above it. There is sometimes an irregular refraction near the horizon, caused by the vapors near the surface of the earth.

Owing to refraction increasing the altitude of a heavenly body, the correction must always be subtracted from the observed altitude.

EXPLANATION WHY TIME IS GAINED IN SAILING EAST, AND LOST IN SAILING WEST.

A difference of longitude causes difference of time, and any place east of another place must have the latest time, because the earth in its revolution turns on its axis from west to east, and the place which is the furthest eastward is sooner carried under the sun, and as an entire rotation, or 360°,

the circumference of the earth, is performed in 24 hours, 15° of longitude must be equal to one hour of time. For example: Greenwich is 74° east of New York; consequently, when it is noon at New York, it is 4 hours and 56 minutes past noon at Greenwich, the sun having passed the meridian of the latter place nearly five hours earlier than at New York. So it is that any place west of another has less time, because the observer does not see the sun rise as early as the observer east of him.

This, in navigation, is called difference of time; and the longitude of any place may easily be deduced by converting this time into degrees and parts of a degree. For this reason, a ship which is sailing eastward around the globe comes to the meridian earlier each day, as her course makes her gain time (shortening each day), while if she were sailing to the westward it would take longer each day for the sun to reach her, and she would lengthen her day, thus losing time; consequently, in the circumference of the globe, one whole day would be gained in sailing to the eastward, and lost in sailing to the westward.

THE GULF STREAM.

The Gulf Stream of the Atlantic is the most important and interesting, as a study, of all the great ocean currents. The following notes on the subject are compiled from reports of Dr. Bache, Superintendent of the Coast Survey.

The ocean, within the region of the Gulf Stream, is divided into several bands of higher and lower temperature, of which the axis of the Gulf Stream is the hottest, the temperature falling rapidly inshore and more slowly outside.

Thus on a line perpendicular to the axis of the stream, drawn from Sandy Hook, the temperature at the depth of 15 fathoms and 100 miles, was 63°; at 150 miles, 67°; at 240 miles, 63° to 80°. The late Lieut. G. M. Bache discovered a band of water so much colder than the rest, that he called it the "Cold Wall," the cold water appearing to confine the hot water as by a wall on the inshore side. Its distance from Sandy Hook is from 230 to 280 miles; its distance from Cape May is between 132 and 178 miles-the thermometer at 15 fathoms on the Sandy Hook section rising from 624° to 80°, or 18° in 50 miles; on the Cape May section, rising from 62° to 834°, or 21° in 46 miles; at Charleston, at the depth of 20 fathoms,

The

rising from 674° to 79° in 15 miles; and at St. Simons, from 70° to 76° in 12 miles, being at the rate of 4 tenths to 9 tenths of a degree to a mile. Besides this remarkable cold band there are two outside ones, sufficiently well defined, though the differences of temperature are less marked, the existence of which should be known to the navigator, that he be not perplexed in crossing the stream and finding warm water, to meet with cold, then warm, and then cold again. The positions of these bands may be somewhat changed when more thoroughly considered. Inside of the "Cold Wall," there is a warm band, and then the cold water of the shore. axis of the stream takes, in general, the curve of the coast, below rather than above the water, being turned to the eastward by the shoals off the southern coast of New England. The axis of the cold band, the minimum of temperature which forms the "Cold Wall," follows the shore and shoals in its bendings more closely than the axis of the Gulf stream, and is traced with considerable probability to longitude 66°. The warm water of the Gulf Stream rests on a cold current flowing toward Cape Florida, the coldest water keeping near the Atlantic coast, below the surface, if not at it. By observations at several points along the coast in 400 fathoms, between Sandy Hook and Cape Florida, the surface temperature exceeding 80°, the thermometer indicated 464° to 55°; off Hatteras in 1000 fathoms, 40°.

The warm water of the Gulf Stream is of very different depths at different points of its course, and in different parts of any one of the sections across it. From the deepest portion in the cross sections the warmer water flows off toward the shore, and outward, overlying the cold. This thins out as it approaches the shore, the cold water which lies at the bottom coming up in the northern sections, but the warm water prevailing to the very shore, and at considerable depths, in the southern. When the cold water is forced up by a bank or shoal, or when it comes to the surface from the thinning out of the warm, there is, of course, a considerable change of temperature. This cold water from the north prevails on the inside of the cold axis, at moderate depths, as far south as Hatteras, and probably to the south of it. Acting-Master Jones found 50 miles S.E. of Charleston light, running to the S. W., the surface water being 75°, and at 20 fathoms 68°, the axis of the Gulf Stream being 82°, moderately warm water extending to the bottom.

The direction of the axis of the stream indicates the set of the current in that band. To the right and left of it the current is outward and onward, and to the left, as far as the "Cold Wall," is inward and onward.

Inside of the "Cold Wall," north of Cape Hatteras, and probably south of it, the current is southerly along the coast.

The velocity of the current in the axis of the stream, on the Cape Canaveral section, is about 3 miles per hour; on the Cape Fear section, about 2 miles per hour; and on the Sandy Hook section, about 1 mile per hour.

In the Charleston section, and to the south, the bands of cold and warm water, with scarcely an exception, are produced by the shape of the bottom. The elevated portions of the bottom, forcing up the cold water into the warm, cause cold streaks, and the division into cold and warm bands.

The variations in temperature in different years and at different seasons are considerable, the more southerly sections in the same season giving usually the highest temperature. But in July, 1846, on the axis of the Gulf Stream, the temperature was higher at Sandy Hook than in June, 1853, at Canaveral by 14°, and higher than at Charleston by 54°.

The low temperatures observed show that the Gulf Stream is comparatively a superficial current on the surface of an ocean of cold water. The temperatures have been observed from the surface to the depth of 500 fathoms-in a few instances as low as 1300 to 1500 fathoms.

Navigators are advised to make their observations at the depth of 20 fathoms. Saxton's metallic thermometer is highly recommended. A common Six's self-registering thermometer, or a common thermometer enveloped in cotton or other bad-conducting material, allowed to remain below the surface long enough to take the temperature, will answer.

TIDES.*

The surface of the ocean rises and falls twice in a lunar day, about 24 h. 52 min. of mean time.

On the coast the tides appear as alternate elevations and depressions of the sea, and also as horizontal movements of the water, alternately flowing and ebbing, and the word "tide" is commonly used to designate both phases of the phenomenon.

In hydrography the term tide signifies only the vertical movement of From The Sailor's Handy-Book," the valuable work of my esteemed friend, Lieutenant Qualtrough, U.S. Navy.

the water, and the words "rise" and "fall" are used with reference to the same motion. "Stand" is the term used to denote the interval of time at high or low water, during which no vertical motion is perceptible. The range of the tide is the height from low water to high water.

The horizontal movement of the water is known as the " tidal current," and the terms "flood" and "ebb" are used to indicate the general direction of the current. "Slack" is the word used to designate the interval of time during which no horizontal motion is perceptible.

The tides do not always rise to the same height, but every fortnight, after the new and full moon, they become much higher than they were in the alternate weeks, or after the first and last quarters of the moon.

These high tides are called "spring tides," and the low ones "neap tides."

The close relation which the times of high water bear to the times of the moon's meridian passage, shows that the moon's influence in raising the tides is much greater than that of the sun.

While the whole attraction of the sun upon the earth far exceeds that of the moon, yet, owing to the greater proximity of the latter, the difference between its attraction at the center of the earth and at the nearest or most remote point of its surface (which difference produces the tides) is about two and a-half times as great as the difference of the sun's attraction at the same points. Though each of these bodies may be supposed to cause two tidal waves, the tides must be regarded as the result of their combined action.

At the time of full and change of the moon, the combined effect produces the spring tides, and high water is then higher, and low water lower than at mean tides. When the moon is in perigee, or nearest the earth, the rise and fall is sensibly increased.

When the moon is in quadrature, or 90° from the sun, the attractions of the two bodies upon the waters act in opposition, and the neap tides are produced.

Very small tides will take place about the time of the earth's perihelion passage, if the moon is in apogee, and also in quadrature.

During the first and third quarters of the lunar month the solar wave lies to the west of the lunar wave, and the combined tide wave will be to the westward of that, due to the moon alone; and this causes an acceleration of the time of high water, commonly called "priming." In the second and fourth quarters the sun's influence acts to retard the lunar wave, and causes what is known as "lagging" of the tides.