636
THEORY AND PRACTICE OK ENGINEERING.
Book II.
as 15 to 10O. It is impossible with our present means to define the height at which thewaters stand, or the level of the ocean, from the constant motion to which it is subjectedfrom winds and currents. Its greatest depth has never been fathomed, and it has rarelybeen sounded beyond a mile ; 800 or 900 fathoms were reached by the sea-clamms bv Cap-tain Parry in latitude 74° 30' north, and 78° 1' west longitude.
The specific gravity of sea-water is the same in nearly all latitudes when examined at adistance, from the discharge of fresh-water poured in by the rivers ; its mean is about1*02757, though, from the impurities it holds in solution, this must necessarily differ; theseconsist of muriate of lime, magnesia, potash, and other matters.
The colour of the sea varies, probably from the different animal and vegetable mattersdiffused through it in a putrescent state ; it is often a blue green, and at the Tropics anazure blue, in the Mediterranean of a beautiful purple tinge.
The temperature differs according to the latitudes and depths, and seasons of the year :at the equator it is from 80° to 82°; at the temperate zones it is higher in the winter thanin summer. The decrease of heat is calculated at about one degree for each degree of lati-tude. The cold increases with the depth in the tropical seas, whilst in higher latitudes thereverse law is observed.
The prevailing currents have two directions; those which originate in the Tropics goround the globe in a western direction, and those of the polar seas in the direction of theequator.
Between thirty degrees north and the same of south latitude, the western current moveswith a velocity of about ten miles a-day ; in the Atlantic it takes two directions, one ofwhich passes to the Cape Verde Islands round the Gulf of Mexico , through the Bahama channel, and along the coasts of North America ; itagain alters its course at Newfoundland ,and proceeds in a south-eastern direction to the Canary Islands , and then joins the streamfrom whence it took its departure.
The North Atlantic Ocean has a current between 11° and 43°, extending 3800 leagues,its velocity increasing as its breadth and depth becomes contracted ; at the Bahama channelits breadth is 51 leagues, and its motion is as much as five miles per hour. This currentreturns to the Azores at the rate of seven or eight miles per day, where its breadth hasbeen computed at 160 leagues. The space comprised between these two currents is stillwater, and is 140 leagues in breadth. These currents exercise a very powerful effect on thecoasts, causing a continual erosion on those that are bold and rugged, whose detritus fallsdown into the ocean, and is carried away to be deposited in less turbulent waters, andperhaps become the foundation of some future island.
The leaves of the sea depend upon the force of the wind, which, by depressing or movinga body of water, at once alters its equilibrium. When water is placed in a bent tube, andmade to ascend and descend alternately, its motion agrees with that of the pendulum,which Newton compared to the action of the waves, and found that their velocity was asthe square roots of their breadths, as taken between the tops of their ridges ; and he alsofound that waves moved through a space equal to their breadth in the same time in whicha pendulum oscillated, whose length was equal to its breadth. Waves, whose breadth are39| inches, will move over that distance in a second of time, and their motion is progressive ;but an object floating on their surface seems to make little way, and their motion doesnot appear to be so considerable.
The tidal currents , which alternately move in opposite directions, exercise a considerabledestroying action on the land, as well as on the formations in progress at the bottom of theocean. The motion of the tides and currents is produced by different means; the first is fromthe influence of the sun and moon, and their height and velocity depend upon the coasts withinwhich they are enclosed. In narrow seas they rise higher, but when the waters meet withno obstruction and can freely expand, they do not exhibit so much elevation. In theBristol Channel , where the passage is narrow*, the tide runs at the rate of 14 miles an hour.In the ocean are permanent currents from 50 to 250 miles in breadth, w*hich constantlyflow* in the same direction, in consequence of the influence of particular winds, or from theexpansion and contraction that the waters of the sea are subjected to when acted upon byheat or cold, w'hich change the condition of their temperature.
The currents towards the Tropics from the poles are produced by the increase of specificgravity of the water as it becomes colder, which occasions it to sink, and thus allow-s thatwhich is warmer, and consequently lighter, to float at the surface. Thus rising and de-scending currents are produced, the low*er parts of the ocean in high latitudes become ofhigher specific gravity than those at the same depth between the Tropics ; the cold waterrushing to occupy the lower place of that more highly rarified, which, in its turn, movesforward in the opposite direction.
The tides are not affected entirely by the moon’s influence ; the sun has considerablepower, which has been estimated at about one-fourth of the whole. When the sun andmoon are in conjunction or opposition, and exert their combined influence in elevating thewaters of the ocean in the same direction, they rise to the height called a spring-tide ; this