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LIFE-PRESERVERS

1067

LI@HT

capsized, can right itself. Lionel Lukin of London, England, patented a lifeboat in 1785, and in 1789 Henry Greathead constructed a better one, inventing a curved keel. Till 1851 Greathead's model remained almost the only lifeboat used. But it could not discharge water, nor right itself when upset. Then James Beeching of Yarmouth constructed the first self-righting boat, and Peake of Woolwich designed another, which was improved by many men and so became the standard. The model lifeboat has great resistance against upsetting; is speedy against a heavy sea; is easily launched; discharges water immediately by relieving-tubes; rights itself if overturned; is exceptionally strong; and carries many people. Its length is 33 feet, its width eight feet, — a great breadth of beam in proportion to the length.

Life»Preser'vers, a buoy or belt designed to be attached, to the person for the preservation of life in shipwreck. They are generally made of cork covered with canvas, or of indiarubber inflated with air. Many varieties of life-preservers have been devised, among which we mention the lifebelt designed by Admiral Ward in 1854. It has four separate compartments, so that if one should be punctured, the belt's buoyant power would not be destroyed. There also are life-preserving jackets; life-floats; annular life-preservers; life-preserving trousers and suits. Life-preservers of many other forms and materials have been demised; and ships are required to carry a sufficient number for the safety of all passengers.

Life=Sav'ing Set'vice. Lighthouses and beacons have been built along the seacoasts of Europe and the United States, and, in addition lifeboat stations have been established, with organized crews for the rescue of all shipwrecked persons. In the United States alone more than 11,000 lives were saved by this service in the first ten years (1871-81) of operation. The shores of the United States — lakes and seas — are over 10,000 miles in extent, and this entire line is divided into 12 districts with 278 stations. Two hundred are on the Atlantic, 60 are on the lakes, and 17 on the Pacific. At many stations the English lifeboat is used, although in general it has been found too heavy for efficient service. The boats chiefly used are light, and can, on their transporting carriages, be easily dragged along the shore by their crews. For projecting a line to a stranded vessel the mortar is generally preferred to the rocket. In addition to the traveling life-buoy a metallic car is used, which will hold a small number of persons, who enter it by a small manhole and are shut in and safely drawn ashore, even though overturned by the surf. This clever contrivance has been the means of rescuing many invalids, children and aged persons.

Light may be roughly defined as that which produces the sensation of sight. Just what light really is will be clearer after we have considered some of the phenomena of light; since from these only can we deduce the nature of light. The fundamental phenomena perhaps are the following:

1. In   any   homogenous   medium   light travels   in   straight   lines. The  strongest evidence for this statement is obtained from the fact that computations, based on the assumption of this fact, invariably lead to correct   results. A  partial   exception   to this rule should be noted in the  case in which light passes through very small openings. Here some of the rays are deflected from a straight line in accordance with the principle of diffraction. See DIFFRACTION.

2. When a ray of light strikes upon a polished surface, the ray is sent off in another direction. This phenomenon is called re-flection. By  numerous   experiments,   it has been found that the angle between the incident ray and the normal to the reflecting surface is always equal to the angle between  the  reflected  ray  and  the  normal. This fact is generally expressed by saying that the angle of incidence is equal to the angle of reflection. It is found also that the angle of reflection lies in the same plane as the angle of incidence. One proof for these two laws of reflection is the fact that the image of any object in a plane surface is of the same size and shape as the object.

3. When a ray of light traveling in one medium strikes the bounding surface of another medium, a part of the light is reflected according to the laws just stated, but another part enters the second medium, and in so doing has its direction also changed. The entering  ray  is  said  to  be  refracted. Thus it is found that a ray passing from the bottom of  a creek to the surface and thence to the eye of an observer is bent away from the  normal  to  the  surface of the creek at the point where it leaves the water. In like manner, if a ray enters the water from the air, it is always bent toward the normal. The first satisfactory description  of   these    phenomena   was   given by Willebrord  Snell   (1591-1626),  and  is  now known  as   Snell's  law. If we  define  the angle  between  the  refracted  ray  and  the normal as the angle of refraction, then Snell's law is that the

sine of angle of incidence

—-?~?-r-:= constant.

sine of angle of refraction

This constant is called the index of refraction for the medium under consideration, and may be denoted by n, so that we may write

sin i sin r

. — n = refractive index,

where i and r denote the angles of incidence and refraction respectively.