others, cases will frequently occur in the adjustment of pecuniary concerns, in which it is desirable to be able to form an estimate of the duration of life, and as it is evidently a subject on which certainty cannot be attained, we must be content with that species of knowledge which rests on probability. This degree of knowledge, which is the limit of our acquaintance, with many other important facts, is, in a comprehensive view of this subject, infinitely more useful and proper than more positive knowledge would be. At whatever period the world was first inhabited, there is undoubted evidence that for at least 3000 years past the general duration of human life has been much the same as it now is; nor has any great difference been observed between the inhabitants of different climates, the negro of Africa (in some instances at least) attaining to as great age as the European. The human frame appears to adapt itself with little difficulty to the atmosphere and local peculiarities of the country in which it is born, or even into which it is afterwards removed. Thus not only the children of persons who have removed from Great Britain to different parts of the continent of North America, but also the emigrants themselves, have been found to live as long as in the former country. Men can live equally well under very different circumstances, it is sudden changes

that are injurious to the human frame; and temperate climates being less liable to such changes are found to be most favourable to the duration of life. There are however in almost every country, particular districts in which the inhabitants are found to live longer than in other situations, which proceeds chiefly from a free circulation of air, uncontaminated by the noxious vapours and exhalations which destroy its purity in other parts; thus hilly districts are almost universally found to furnish more instances of long life, than low and marshy situations.

The knowledge of the duration of human life in general, and of its probable continuance at all ages, has been ascertained with sufficient correctness for all practical purposes from the observations which have been made on the bills of mortality of different places. Dr. Halley formed a table of the probabilities of life from the registers of the births and burials of the inhabitants of the city of Breslaw, the capital of the duchy of Silesia in Germany, from the year 1687 to 1691. A similar table was formed by Mr. Thomas Simpson from the London bills of mortality, from 1728 to 1737; and other tables of the same kind have been since published by M. Dupré de St. Maur, M. Kerseboom, M. de Parcieux, Dr. Price, and others, from which the following are selected.

In order to find the expectation of life at any age, from a table, like the above, which shows the number that die annually at all ages, divide the sum of all the living in the table, at the age whose expectation is required and at all greater ages, by the sum of all that die annually at that age and above it; or, which is the same, by the number of the living at that age; and half unity subtracted from the quotient will give the expectation required. Thus, at the age of 65, the sum of all the living at that and all greater ages, is 18,580; the number living at that age is 1,652; and the former number divided by the latter, and half unity subtracted from the quotient, gives 10.88 for the expectation of the age of 65. In this manner the following table is formed.

Among any considerable number of lives selected from the common mass, such as the nominees to a tontine, or the members of an assurance or annuity society, the duration of life will always be found greater than it is represented by tables formed from general bills of mortality. Thus, M. Kersseboom found that among the state annuitants in Holland 1 in 14 lived to upwards of 80 years of age, and the nominees to the life annuities granted by the governments of France and Great Britain have been found to live longer than the duration given by any table formed from bills of mortality. In some few country situations, where the injurious habits and artificial mode of living which prevail in large cities have made little progress, the duration of life has been found unusually great; thus at Ackworth in Yorkshire 1 in 14 died turned of 80 years of age; and according to an account of the parish of Kingham in New England, in the first volume of "Memoirs of the American Academy," the number of deaths in 54 years had been 1113, of which 1 in 13 had survived 80 years.

Life

LIFE annuities. See ANNUITIES. annuities secured by land, differ from those already described only in this, that the annuity is to be paid up to the very day of the death of the age in question, or of the person upon whose life the annuity is granted. To obtain the more exact value therefore of such an annuity, a small sum must be added to the same as computed by the rules in the article ANNUITIES, which will be different according as the payments are yearly, halfyearly, or quarterly. Dr. Price has entered at large on the subject, and according to him the addition is,

LIFE estates, or estates for life, are of two kinds; either such as are created by the act of the parties, or such as are created by the operation of law, as estates by the curtesy or dower. Estates for life, created by deed or grant, are, where a lease is made of lands or tenements to a man, to hold for the term of his own life, or for that of another person, or for more lives than one; in any of which cases he is called tenant for life, only when he holds the estate by the life of another, he is usually termed tenant pur auter vie, for another's life. Estates for life may be created not only by the express terms before mentioned, but also by a general grant, without defining or limiting any spe cific estate. Where estates are granted for the lives of others, and they absent themselves seven years, and no proof is made of their being in existence; in any action commenced for the recovery of such tenements by the lessors or reversioners, they shall be accounted as dead, and the jury shall give their verdict accordingly; (19 Charles II. c. 6.) and, on application to the Chancellor, the party holding such estates may be compelled to produce the persons on whose lives such estates depend.

LIGAMENT, in anatomy, a strong compact substance, serving to join two bones together.

A ligament is more flexible than a cartilage, not easily ruptured or torn, and does not yield, or at least very little, when pulled.

LIGHT, is that principle or thing by which objects are made perceptible to our sense of seeing; or the sensation occasioned in the mind by the view of luminous obs jects. The nature of light has been a subject of speculation from the first dawnings of philosophy. Some of the earliest philosophers doubted whether objects became visible by means of any thing proceeding from them, or from the eye of the spectator; but this opinion was qualified by Empedocles and Plato, who maintained, that vision was occasioned by particles continually flying off from the surfaces of bodies, which met with others proceeding from the

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eye; while the effect was ascribed by Pythagorus solely to the particles proceeding from the external objects, and entering the pupil of the eye. But Aristotle defines light to be the act of a transparent body, considered as such; and he observes, that light is not fire, nor yet any matter radiating from the luminous body, and transmit ted through the transparent one.

The Cartesians have refined considerably on this notion; and hold that light, as it exists in the luminous body, is only a power or faculty of exciting in us a very clear and vivid sensation; or that it is an invisible fluid present at all times and in all places, but requiring to be set in motion by a body ignited or otherwise properly qualified to make objects visible to us.

Father Malbranche explains the nature of light from a supposed analogy between it and sound. Thus he supposes all the parts of a luminous body are in a rapid motion, which, by very quick pulses, is constantly compressing the subtle matter between the luminous body and the eye, and excites vibrations of pression: as these vi brations are greater, the body appears more luminous; and as they are quicker or slower, the body is of this or that colour. The Newtonians maintain, that light is not a fluid, but consists of a great number of very small particles, thrown off from the luminous body by a repulsive power, with an immense velocity, and in all directions. And these particles, it is also held, are emit ted in right lines: which rectilinear motion they preserve till they are turned out of their path by some of the following causes, viz. by the attraction of some other body near which they pass, which is called inflection, or by passing obliquely through a medium of different density, which is called refraction; or by being turned aside by the opposition of some intervening body, which is called reflection; or lastly, by being to tally stopped by some substance into which they penetrate, and which is called their extinction. A succession of these particles following one another, in an exact straight line, is called a ray of light; and this ray, in whatever manner its direction may be changed, whether by refraction, reflection, or infiection, always preserves a rectilinear course, till it be again changed; neither is it possible to make it move in the arch of a circle, ellipsis, or other curve. For the above properties of the rays of light, see the several words REFRACTION, REFIAC, TION, &c.

The velocity of the rays of light is truly astonishing, amounting to nearly two hundred thousand miles in a second of time, which is about a million times greater than the velocity of a cannon ball. And this amazing motion of light has been manifested in various ways, and first from the eclipses of Jupiter's satellites. It was first observed by Roemer, that the eclipses of those satellites happen sometimes sooner, and sometimes later, than the times given by the tables of them; and that the observation was before or after the computed time, according as the earth was nearer to, or further from Jupiter, than the mean distance. Hence Roemer and Cassini both concluded, that this circumstance depended on the distance of Jupiter from the earth; and that, to account for it, they must suppose that the light was about fourteen minutes in crossing the earth's orbit. This conclusion, however, was afterwards abandoned, and attacked by Cassini himself: but Roemer's opinion found an able advocate in Dr. Halley, who removed Cassini's difficulty, and left Roemer's conclusion in its full force.

It has since been found, by repeated experiments, that when the earth is exactly between Jupiter and the sun, his satellites are seen eclipsed eight minutes and a quarter sooner than they could be according to the tables; but when the earth is nearly in the opposite point of its orbit, these eclipses happen about eight minutes and a quarter later than the tables predict them. Hence, then, it is certain that the motion of light is not instantaneous, but that it takes up about sixteen minutes and a half of time to pass over a space equal to the diameter of the earth's orbit, which is at least one hundred and ninety millions of miles in length, or at the rate of near two hundred thousand miles per second, as above-mentioned.

Hence, therefore, light takes up about eight minutes and a quarter in passing from the sun to the earth; so that, if he should be annihilated, we should see him for eight minutes and a quarter after that event should happen; and if he were again created, we should not see him till eight minutes and a quarter afterwards. Hence also it is easy to know the time in which light travels to the earth, from the moon, or any of the other planets, or even from the fixed stars, when their distances shall be known; these distances are, however, so immensely great, that from the nearest of them, supposed to be Sirius, the dog-star, light takes up many years to travel to the earth: and

it is even suspected, that there are many stars whose light has not yet arrived at us since their creation. And this, by-the-bye, may perhaps sometimes account for the appearance of new stars in the heavens. Our excellent astronomer, Dr. Bradley, after- ́ wards found nearly the same velocity of light as Roemer, from his accurate observations, and most ingenious theory, to account for some apparent motions in the fixed stars; for an account of which see ABERRATION of light. By a long series of these observations, he found the difference between the true and apparent place of several fixed stars, for different times of the year; which difference could no otherwise be accounted for, than for the progressive rays of light. From the mean quantity of this difference he ingeniously found, that the ratio of the velocity of light to the velocity of the earth in its orbit, was as 10,313 to 1, or that light moves 10,313 times faster than the earth moves in its orbit about the sun; and as this latter motion is at the rate of 18 miles per second nearly, it follows that the former, or the velocity of light, is at the rate of about 195,000 miles in a second; a motion according to which it will require just 8' 7" to move from the sun to the earth, or about 95,000,000 of miles.

It was also inferred, from the foregoing principles, that light proceeds with the same velocity from all the stars. And hence it follows, if we suppose that all the stars are not equally distant from us, as many arguments prove, that the motion of light, all the way it passes through the immense space above our atmosphere, is equable or uniform. And since the different methods of determining the velocity of light, thus agree in the result, it is reasonable to conclude, that in the same medium, light is propagated with the same velocity after it has been reflected as before. For an account of Mr. Melville's hypothesis of the different velocities of differently coloured rays, see COLOUR.

To the doctrine concerning the materiality of light, and its amazing velocity, several objections have been made, of which the most considerable is; that as rays of light are continually passing in different directions from every visible point, they must necessarily interfere with each other in such a manner as entirely to confound all distinct perception of objects, if not quite to destroy the whole sense of seeing; not to mention the continual waste of substance,

which a constant emission of particles must occasion in the luminous body, and thereby, since the creation, must have greatly diminished the matter in the sun and stars, as well as increased the bulk of the earth and planets, by the vast quantity of particles of light absorbed by them in so long a period of time. But it has been replied, that if light were not a body, but consisted in mere pression or pulsion, it could never be propagated in right lines, but would be continually inflected ad umbram. Thus, Sir Isaac Newton: "A pressure on a fluid medium, i. e. a motion propagated by such a medium, beyond any obstacle, which impedes any part of its motion, cannot be propagated in right lines, but will be always inflecting and diffusing itself every way, to the quiescent medium beyond that obstacle.

The power of gravity tends downwards; but the pressure of water arising from it tends every way with an equable force, and is propagated with equal ease and equal strength, in curves as in straight lines. Waves, on the surface of the water, gliding by the extremes of any very large obstacle, inflate and dilate themselves, still diffusing gradually into the quiescent water beyond that obstacle. The waves, pulses, or vibrations of the air, wherein sound consists, are manifestly inflected, though not so considerably as the waves of water; and sounds are propagated with equal ease through crooked tubes and through straight lines; but light was never known to move in any curve, nor to inflect itself ad umbram."

that

It must be acknowledged, however, many philosophers, both English and foreigners, havę recurred to the opinion, that light consists of vibrations propagated from the luminous body, through a subtle ethereal medium.

Dr. Franklin, in a letter dated April 23, 1752, expresses his dissatisfaction with the doctrine, that light consists of particles of matter continually driven off from the sun's surface, with so enormous a swiftness. "Must not," says he, "the smallest portion conceivable have, with such a motion, a force exceeding that of a twenty-four pounder discharged from a cannon? Must not the sun diminish exceedingly by such a waste of matter; and the planets, instead of drawing nearer to him, as some have feared, recede to greater distances, through the lessened attraction? yet these particles, with this amazing motion, will not drive be fore them, or remove, the least and slight

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