slightly lengthened nose, two beards at the lower jaw, and the first ray of the ventral fins elongated into a bristle. Its colour is a ferruginous brown.

LONG (ROGER), D. D. Master of Pembroke-hall in Cambridge, Lowndes's professor of astronomy in that university, &c. was author of a well known and much approved treatise of astronomy, and the inventor of a remarkably curious astronomical machine. This was a hollow sphere of 18 feet diameter, in which more than 30 persons might sit conveniently. Withinside the surface, which represented the hea vens, was painted the stars and constellations, with the zodiac, meridians, and axis parallel to the axis of the world, upon which it was easily turned round by a winch. He died December 16, 1770, at 91 years of age.

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A few years before his death, Mr. Jones gave some anecdotes of Dr. Long, as follows: "He is now in the 88th year of his age, and for his years vegete and active. He was lately put in nomination for the office of vice-chancellor : he executed that trust once before, I think in the year 1737. He is a very ingenious person, and sometimes very facetious. At the public commencement, in the year 1713, Dr. Greene (master of Bennet College, and afterwards Bishop of Ely) being then vice-chancellor, Mr. Long was pitched upon for the tripos performance it was witty and humorous, and has passed through divers editions. Some that remembered the delivery of it, told me, that in addressing the vice-chancellor, (whom the university wags usually styled Miss Greene) the tripos orator, being a native of Norfolk, and assuming the Norfolk dialect, instead of saying, 'Domine vice-cancellarie,' archly pronounced the words thus, Domina vice-cancellaria;' which occasioned a general smile in that great auditory. His friend, the late Mr. Boufoy, of Ripton, told me this little incident: That he and Dr. Long, walking together in Cambridge, in a dusky evening, and coming to a short post fixed in the pavement, which Mr. Boufoy, in the midst of chat and inattention, took to be a boy standing in his way, he said in a hurry, 'Get out of my way, boy.' That boy, sir,' said the Doctor, very calmly and slyly, is a post-boy, who turns out of his way for nobody.' I could recollect several other ingenious repartees, if there were occasion. One thing is remarkable, he never was a hale and hearty man, always

of a tender and delicate constitution, yet took great care of it; his common drink water; he always dines with the fellows in the hall. Of late years he has left off eating flesh-meats; in the room thereof puddings, &c. sometimes a glass or two of wine."

LONGEVITY, the continuance of life beyond its ordinary period of duration. The term of human life does not in general much exceed 80 years, but it is well known that instances occasionally occur of persons living to the age of 100 years and upwards. Such instances however have not excited that general attention, which from the nature of the subject might be expected, and it is only of late years that any extensive collection of them has been formed, or at tempts made to ascertain the circumstances and situations in which the different individuals preserved their lives to an age so much beyond the usual lot of man. The most extensive catalogue of this kind, is that published by J. Easton, which, though very defective, contains the names and some particulars of 1712 persons who had attained to a century and upwards, having died at the following ages:

from 100 to 110 years...... 1310

The circumstances which chiefly tend to promote longevity may be reduced to the following heads:

1. Climute. A large majority of the recorded instances of great age were inhabi tants of Great Britain or Ireland, of France, Germany, or the north of Europe, from which it appears that moderate or even cold climates are the most favourable to long life. Heat relaxes and enfeebles, while cold consolidates and strengthens the hu man frame. The diet also of hot countries is less nourishing than that of cold ones; and there is generally a greater disposition, and greater opportunities to indulge in va rious excesses in the former, than in the lat

ter.

There are however a few instances of natives of very hot climates having attained to great age, but they have been chiefly negroes in the West Indies and America, whose ages were probably not very cor rectly ascertained.

2. Parentage. Being born of healthy parents, and exempted from hereditary disease, are circumstances evidently favourable to the duration of life; and numerous instances warrant the opinion, that longevity prevails in some families more than in others, or that descent from longlived ancestors is one of the circumstances which give the greatest probability of attaining to extreme old age.

3. Form and size of the individual. It is generally admitted, that persons of a compact shape, and of a moderate stature, are the most likely to live long. Tall persons frequently acquire a habit of stooping, which contracts the chest, and is a great impediment to free respiration; whereas the short sized find little difficulty in keeping themselves erect, and are naturally much more active, by which the animal functions are retained in a state of greater perfection; the only disadvantage attending a short stature is, that it is frequently accompanied with corpulence, which is rather unfavourable to long life.

4. Disposition of Mind. Nothing is more conducive to longevity than to preserve equanimity and good spirits, and not to sink under the disappointments of life, to which all, but particularly the old, are necessarily subjected. This is a point which cannot be too much inculcated, as experience continually shows that many perish from despondency, who, if they had preserved their spirits and vigour of mind, might have survived many years longer. Neither the irritable, who are agitated by trifles, nor the melancholy, who magnify the evils of life, can expect to live long. Even those who suffer their strength and spirits to be exhausted by severe study, or other mental exertions, seldom reach great age. In the list before referred to, of 1712 persons who lived about a century, Fontenelle (who did not quite reach 100 years) is the only author of any note; and his great age is ascribed to the tranquil ease of his temper, and that liveliness of spirits for which he was much distinguished. Among those who have devoted themselves to the study or practice of music, a profession which encourages cheerfulness of mind, instances of great age have been very frequent.

5. Occupation. No person that leads an idle life will ever attain to great age; but health and long life must depend much on the manner in which the individual is employed. Those occupations are certainly the most conducive to the duration of life,

which are carried on in the open air, and require activity or labour; thus farmers, gardeners, and labourers in the country, are in general the longest lived. Foot soldiers also, who have survived the dangers of war, are remarkable for long life: they are generally stout and vigorous men, and the regularity to which surviving soldiers must have accustomed themselves, whilst their careless and disorderly companions have/ dropped off, the erect posture to which they have been trained, and being of course men well formed by nature, and habituated to walk well (by which they enjoy the most natural exercise in perfection) all combine in their favour. Sailors also would furnish many instances of longevity if comfortably provided for in their old age: of this a striking proof is given in the accounts drawn up by Dr. Robertson of the pensioners in Greenwich Hospital. In the year 1801, the complement of in-pensioners was 2410, of whom there were 96 of the age of 80 years and upwards; of this number 13 were above 90 years of age, and one man 102 years old. The number of out-pensioners was about 2500, of whom it appeared there were only 23 from 80 years of age and upwards. Of the former therefore about 4 in 100 survived 80 years of age, but of the latter not 1 in 100 attained that age, a sufficient evidence of the benefits of regularity and ease in the advanced period of life, and of the attention paid to the health of the in-pensioners at that excellent institution.

6. Mode of Living. If persons were to live with the simplicity of ancient times, it is probable that they would attain long life, without experiencing any material illness, merely by a proper attention to air, exercise, clothing, and diet. But in the present state of society, the great bulk of the community follow, not a natural, but an ar tificial mode of life, and thence are per petually exposed to various temptations, which they find it difficult always to resist, and to dangers which they cannot always avoid. Most persons however have it in their power in some degree to regulate their manner of living by their own choice; and by a little attention to their food, clothing, employment, rest, and temper of mind, might not only contribute materially to the prolongation of their lives, but preserve themselves from many diseases, and greatly increase their relish for all the enjoyments of life.

The importance of wholesome food, for

the preservation of health and promoting long life, and the avoiding of excess, whether in eating or drinking, is sufficiently obvious. Some instances, indced, are recorded of persons who have continued to commit excesses, and have lived long; but these are to be considered in no other light than as exceptions to a general rule; and it may reasonably be contended, that if such persons lived to a great age, notwithstanding their intemperance, they would have lived much longer had they followed a different course. Experience will point out those articles of food which are best adapted to the constitution of each individual, and there cannot be a better rule than to adhere to them as far as circumstances will permit. It may be observed, however, that people in general, especially those who do not labour, eat much more than nature requires; that a little abstinence or selfdenial may often be of use, either to prevent or to cure disease; and at any rate, that none but hard working people, the young who are growing fast, or persons who are travelling about, should eat more than one full meal each day.

As to clothing, much must depend on si tuation and climate; but it is generally found a useful practice to wear woollens next the skin. It is remarked in many parts of Scotland, that since the use of flannel shirts has been given up by the lower orders, the rheumatism and other diseases formerly unknown, have become very frequent, and are daily increasing. In the West India islands, if care be taken to make the troops wear flannel shirts, they are generally exempt from various disorders, which otherwise would probably have attacked them. Even the negroes themselves. are said to prefer flannel to cotton or linen, and find it a much more comfortable and useful dress.

Exercise cannot be too much recommended; and as the inhabitants of large towns, and persons engaged in sedentary occupations, cannot take all the exercise abroad that may be necessary for their health, they ought as much as possible to accustom themselves to be walking about even in their own house, for thongh this practice does not make up for the want of exercise abroad, it is certainly the best substitute for it. Exercise is attended with the advantage of creating an incination to retire early to rest, and of inducing sound sleep. Every one should take all the repose that nature requires, but should never

continue long in bed without sleeping. Early rising, even if carried to an extreme, is far more conducive to health and long life, than late hours at night and slumbering in bed in the morning.

There is nothing that can tend more to long life than for a person to obtain a complete command of his passions, and in particular to preserve his mind from being ruffled by the occurrences of life. Perhaps there is no maxim more likely to promote good health, and consequently the duration of life, than that of paying a proper attention to temper, temperance, and sleep. By good temper the mind is preserved from disease; and by temperance, the body; and both the mind and the body, when exhausted, are again recruited and restored to their former strength, by a sufficient quantity of repose.

LONGIMETRY, the art of measuring lengths, both accessible, as roads, &c. and inaccessible, as arms of the sea, &c. See SURVEYING.

LONGITUDE of a star, in astronomy, an arch of the ecliptic, intercepted between the beginning of Aries and the point of the ecliptic cut by the star's circle of longitude. See CIRCLE, &c.

LONGITUDE of a place, in geography, is an arch of the equator intercepted between the first meridian, and the meridian passing through the proposed place; which is always equal to the angle at the pole, formed by the first meridian and the meridian of the place.

The first meridian may be placed at pleasure, passing through any place, as London, Paris, Teneriffe, &c. but among us it is generally fixed at London, or rather Greenwich, and the longitudes counted from it will be either east or west, according as they lie on the east or west side of that meridian. The difference of longitude between two places upon the earth is an arch of the equator comprehended between the two meridians of these places; and the greatest possible is 180 degrees, when the two places lie on opposite meridians.

Since the parallels of latitude always decrease, the nearer they approach the pole; it is plain, a degree upon any of them must be less than a degree upon the equator, in the ratio of the co-sine of the latitude to the radius. Hence, as the radins is to the co-sine of any latitude; so is the minutes of difference of longitude between two meridians, or their difference in miles upon the equator, to the distance of these two meri

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LONGITUDE, in navigation, the distance of a ship or place, east or west, from another, reckoned in degrees of the equator. As the discovery of a method to find the longitude would render voyages safe and expeditious, and also preserve ships and the lives of men, the following rewards have been offered by act of parliament, as an encouragement to any person who shall discover a proper method for finding it out: the author or authors of any such method shall be entitled to the sum of 10,000l. if it determines the longitude to one degree of a great circle; to 15,000l. if it determines the same to two-thirds of that distance; and to 20,000l. if it determines the same to onehalf of the same distance; and that half of the reward shall be due and paid when the commissioners of the navy, or the major part of them, agree that any such method extends to the security of ships within 30 geographical miles of the shores, which are places of the greatest danger; and the other half, when a ship, by the appointment of the said commissioners, or the major part of them, shall thereby actually sail over the VOL. IV.

ocean, from Great Britain to any such port in the West Indies as those commissioners, or the major part of them, shall choose for the experiment, without losing their longitude beyond the limits before mentioned. The French, Dutch, Spaniards, and other nations, have likewise offered rewards for the same purpose.

Since, by the motion of the earth round its axis, every point upon its surface describes the circumference of a circle, or 360°, in twenty-four hours time, it is plain it must describe 15° in one hour, because 36015. Hence the difference of longitude may be converted into time, by allowing one hour for every 15 degrees, and proportionally for minutes; also difference of time may be converted into difference of longitude by allowing 15° for every hour, and proportionally for a greater or less time. Consequently by knowing the one we can easily find the other.

Whatever contrivance, therefore, shows the hours of the day, at the same absolute point of time, in two different places, likewise serves to find the difference of longitude between those places. Now, since an eclipse of the moon proceeds from nothing else but an interposition of the earth between her and the sun, by which means she is prevented from reflecting the light she would otherwise receive from the sun, the moment that any part of her body begins to be deprived of the solar rays, it is visible to all those people who can see her at the same time; whence, if two or more different people, at two or more different places, observe the times when it first began or ended, or note the time when any number of digits was eclipsed, or when the shadow begins to cover or quit any remarkable spot, the difference of those times (if there be any) when compared together, will give the difference of longitude between the places of observation.

The longitudes of places may also be determined from the observations of solar eclipses, but these being encumbered with the considerations of parallaxes, are not near so proper as those of the moon; and each of these happening but rarely, another excellent expedient has been thought of, and that is the eclipses of Jupiter's satellites.

Now as neither Jupiter nor any of his attendants have any native light of their own, but shine with a borrowed light from the sun, it happens that each of these, in every revolution about Jupiter, suffers two eclipses, one at their entrance into the sha

M

sage behind his body; whence in each revolution of the satellite there are four remarkable appearances, by the observation of any one of which the business may be done, viz. one at the entrance into the shadow, and one at the emersion out of it; one at the entrance behind the body, and another at the coming out; but the latter of these, viz. the ingress and egress of the satellite, into and from under the body, is not so much regarded by astronomers as the immersion into and out of the shadow, because, in the former, the difficulty of pronouncing the exact time is very great, it requiring, in each observer, eyes equally good and strong, and telescopes equally large; but the observation of the former of these, viz. the immersion into, and emersion out of the shadow, is easy and practicable, because the quick motions of the satellites plunge them so quickly into the shadow of Jupiter, that it is no difficult matter to pronounce, by any telescope by which they may be seen, the exact time of their immersion and emersion, as any one may soon be satisfied, if he will but try the experiment.

And as each of these happens at the same moment of absolute time, if two or more persons, in different places, note the time of observation, these, when compared together, will give the difference of longitude between the two places of observation. And, when we consider the great number of these eclipses that happen every year, there being more visible in one year than there are days in it, and consequently, but few nights when Jupiter may be seen, (and which is near eleven months of the year) but that an eclipse of one or other happens, and sometimes two or three in a night; the ease with which they may be made, requiring only a telescope of eight or ten feet in length, which may be almost managed with the hand; and the little likelihood there is of missing the times of ingress or egress, they being in a manner momentaneous; and lastly, the great exactness to which they would give the difference of longitude, it being certainly as exact as the latitude can at present be taken; it is much to be wondered at, that the more skilf:l part of our seamen have so long neglected them, and especially in the several ports into which they sail. The eclipses of Jupiter's satellites, and their configurations, are given in the nautical ephemeris.

Besides these, there is another method

and that is, the appulses of the moon to certain fixed stars, and their occultations by the interposition of her body; for, the moon finishing her revolution in the space of twenty-seven days, seven hours, forty. three minutes, there are but few clear nights when the moon does not pass over or so near to some fixed star, that her distance from it, or the time of her visible conjunction with it, may be easily observed by the telescope, and micrometer only; and these, when compared together, or with the visible time computed to the meridian of some place, will show the difference of longitude of those places.

It is a great objection to the methods here described that the agitation of a ship at sea prevents their being useful. But the invention of Hadley's quadrant and its modern improvements, with the degree of perfection to which the moon's place can now be had, by computation, added to the great facilities afforded by the nautical almanack and requisite tables, published by the commissioners of longitude, and other works, particularly Mendoza's extensive Tables, patronized by them, have rendered the determination of the longitude at sea, a thing of easy and general practice by obser vations of the angular distance of the moon from a fixed star. This was first proposed by John Warner, in his Notes to Ptolemy's Geography, in 1514, and since by others, particularly our Sir Jonas Moor, Flamstead, Halley, Bradley; and in later times, with great diligence, zeal, and ability, by the present Astronomer Royal, Dr. Maskelyne. For the processes and computations the reader will have recourse to the works

just mentioned. The principle is simple and easy. An observer at sea measures the angle between the moon and the sun, or a fixed star, while two other observers take their altitudes in order to determine the quantities of refraction and parallax. The two zenith distances, and the oblique distance, constitute a spherical triangle; of which the angle of the zenith may be deter mined, and then by correcting the altitudes for parallax and refraction two other zenith distances may be had, which are correct, and with these and the angle at the zenith, a new triangle is constituted, of which the oblique side is the correct distance. By comparing this distance with those in the nautical almanack, the time at Greenwich is obtained, and the difference between this and the time (observed by an altitude or otherwise) at the ship, gives the differ