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face, to prevent the reflection from taking place.

"In an uniform state, that is to say, by supposing that the density and thickness of the dilated stratum are constant, and the temperature of the superior stratum invariable, the greatest possible angle under which the rays of light could be thus reflected, would be determined with precision: for the largeness of this angle depends entirely on the immediate connection between the sinus of the angles of incidence and reflection for the two media. But of all the rays reflected, those which form the greatest angle with the horizon, appear to issue from the nearest point to that where the phenomenon com

mences.

"In an uniform state then, the point at which the phenomenon commences, is always at a certain distance from the observer: accordingly, if he should happen to advance, the spot at which the inundation seems to begin, appears to advance also in the same direction, and with the same celerity. If the line of march should, therefore, be directed towards a village, which appears to be situated in the midst of the inundation, the limits of the inundation will seem to recede insensibly from this village, and are soon after seen to stretch beyond it.

"When the sun is near the horizon at his rising, the earth cannot have been sufficiently warmed; and at his setting it has become too cold for the phenomenon of the mirage. It then appears to be extremely difficult to perceive both a direct and reflected image of the sun, on account of the elevated temperature of the inferior stratum of the atmosphere. But during the second quarter of the moon,

that planet rises in the afternoon, a time when circumstances are more favourable to the mirage. If it then happens that the brilliancy of the sun and the clearness of the atmo sphere should permit the moon to be discerned at her rising, two images of that planet will be observed, one above the other, in the same vertical line. This phenomenon is known by the name of paraselene.

"The transparency of the sea allows the rays of the sun to pene. trate to a considerable depth; its surface, from its exposition, does not, however, become near so warm as an arid soil in the same circumstances; it cannot communicate, therefore, to the stratum of air that reposes upon it so elevated a temperature; and on this account the mirage is not so common at sea as in the Desert. But the elevation of the temperature is not the sole cause which, under a constant pressure, may dilate the inferior stratum of the atmosphere. In fact, the air possesses the faculty of dissolving water, so far, even as to attain the point of saturation without losing its transparency; and Saussure hath proved that the specificweight of the air decreases in proportion to the quantity of water kept in dissolution. When, therefore, any wind at sea is not impregnated with water, the inferior stratum of the atmosphere, which is in contact with the surface of the ocean, dissolves water anew, and thus becomes dilated. This cause, added to the slight augmentation of the temperature, may, however, produce a state of things favourable to the mirage, which mariners, indeed, frequently observe.

"This last cause, or, in other words, the dilatation of the inferior stratum of the atmosphere, oc

casioned

casioned by the dissolution of a large quantity of water, may take place every moment of the day, as well when the sun is near the horizon, as when he approaches the meridian. It is possible that a parhelion might then be produced, a phenomenon in consequence of which, either at the rising or setting of the sun, one beholds two images of that luminous body above the apparent horizon at the same time. But I never had occasion to observe this latter phenomenon, which is very unfrequent, or to remark the circumstances that accompany it. I therefore propose this latter explanation with a certain degree of reserve, and merely with an intent of furnishing the means of making useful observations,

ADDITION.

"Sincereading the above memoir, I have had frequent opportunities of observing the mirage at land; this has occurred in various seasons, as well as under different circumstances, and the result, even including the minutest details, has always been in conformity to the explanation already given by me. Of all these observations, there is one only, the relation of which may prove useful here.

"I was along with general Bonaparte in the valley of Suez, when he discovered the canal that for merly united the Red Sea with the Mediterranean. This valley, which is some leagues in length, is bounded on the east by that chain of mountains that extends from Syria to Mount Sinai; and on the west by the mountains of Egypt. These mountains are in general sufficiently elevated to exclude the rays of light transmitted by the inferior parts of the sky, and such of the rays as are not thus intercepted,

reach the earth under too large an angle to be reflected by the inferior dilated stratum of the atmosphere. Thus in the hottest part of the day one does not perceive the reflected surface of any portion of the sky, nor is the appearance of an inundation any where to be seen.

"Notwithstanding this, the ef fect of the mirage is not entirely lost the visible objects placed generally on an ascent, whose posi tion corresponds with the inferior parts of the sky, the image of which reflects itself, participates in the effect, although in a less striking manner, on account of their small extent, and also with less force, because the colour is far more obscure than that of the sky. Independently of the representation produced by the direct rays of light, the rays which have emanated from these objects, and are directed towards the earth, become reflected by the inferior stratum of the air, in the same manner as the rays proceeding from the inferior parts of the atmosphere, of which they occupy the place; thus pro ducing a second image of these objects inverted, and placed vertically above the former.

"This duplication of images produces optical illusions, against which it is proper to be on our guard in a desert that may be occupied by an enemy, while no one is at hand to give information relative to such alarming appearances.

"I shall take advantage of this opportunity to mention another op. tical phenomenon, which is not suf ficiently interesting to be made the subject of a particular memoir.

"During our return from Egypt, when we approached the European climates, one morning, a few mi nutes after sun-rise, the sky was clear towards the east; it rained at

the

the same time in a western direc- less than half the circumference:

tion, and we beheld two ordinary rainbows; the one, interior, produced by a single reflection of the rays of light within the space where the rain fell; the other, exterior, occasioned by two reflections. At this moment both the sea and the atmosphere were perfectly calm, and the surface of the water, which was very smooth, reflected the image of the sun in a very discernible manner. This reflected image also gave rise to two other rainbows. The two first-mentioned, which were produced by direct and descending rays, formed segments

the two others, occasioned by the reflected and ascending rays, on the contrary, presented segments greater than 180°. Of these four simultaneous rainbows, the analogous ones had the same basis, and diverged like two segments of the same circumference of a circle, which had reverted on their common chord.

"This phenomenon is only remarkable on account of the circum stances which gave rise to it; and it is sufficient to have mentioned it here."

Of the GENERAL CHECKS to POPULATION.

[From Mr. MALTHUS'S ESSAY on the PRINCIPLE of POPULATION.]

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"The preventive check, is peculiar to man, and arises from that distinctive superiority in his reasoning faculties, which enables him to calculate distant consequences. Plants and animals have apparently no doubts about the future support of their offspring. The checks to their indefinite increase, therefore, are all positive. But man cannot look around him, and see the distress which frequently presses upon those who have large families-he cannot contemplate his present possessions or earnings, which he now nearly consumes himself, and calculate the amount of each share, when with very little addition they 1803.

must be divided, perhaps, among 'seven or eight, without feeling a doubt, whether if he follow the bent of his inclinations, he may be able to support the offspring which he will probably bring into the world. In a state of equality, if such can exist, this would be the simple question. In the present state of society other considerations occur. Will he not lower his rank in life, and be obliged to give up in great measure his former society? Does any mode of employment present itself by which he may reasonably hope to maintain a family? Will he not at any rate subject himself to greater difficulties, and more severe labour than in his single state? Will he not be unable to transmit to his children the same advantages of education and improvement which he had himself possessed? Does he even feel secure that, should he have a large family, his utmost exertions can save them

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rags, and squalid poverty, and their consequent degradation in the community? And may he not be reduced to the grating necessity of forfeiting his independence, and of being obliged to the sparing hand of charity for support?

"These considerations are calculated to prevent, and certainly do prevent, a great number of persons in all civilised nations from pursuing the dictate of nature in an early attachment to one woman.

"If this restraint do not produce vice, as in many instances is the case, and very generally so among the middle and higher classes of women, it is undoubtedly the least evil that can arise from the principle of population. Considered as a restraint on an inclination, otherwise innocent, and always natural, it must be allowed to produce a certain degree of temporary unhappiness; but evidently slight, compared with the evils which result from any of the other checks to population.

When this restraint produces vice, as it does most frequently among men, and among a numerous class of females, the evils which follow are but too conspicuous. A promiscuous intercourse to such a degree as to prevent the birth of children, seems to lower in the most marked manner the dignity of human nature. It cannot be without its effect on men, and nothing can be more obvious than its tendency to degrade the female character, and to destroy all its most amiable and distinguishing characteristics. Add to which, that among those unfortunate females with which all great towns abound, more real distress and aggravated misery are perhaps to be found, than in any other department of human life.

"When a general corruption of morals, with regard to the sex, pervades all the classes of society, its effects must necessarily be, to poison the springs of domestic happiness, to weaken conjugal and parental affection, and to lessen the united exertions and ardour of parents in the care and education of their children; effects, which cannot take place without a decided diminution of the general happiness and virtue of the society; particularly, as the necessity of art in the accomplishment and conduct of intrigues, and in the concealment of their consequences, necessarily leads to many other vices.

"The positive checks to popula tion are extremely various, and include every cause, whether arising from vice or misery, which in any degree contributes to shorten the natural duration of human life. Under this head, therefore, may be enumerated, all unwholesome occu pations, severe labour and exposure to the seasons, extreme poverty, bad nursing of children, great towns, excesses of all kinds, the whole train of common diseases and epidemics, wars, pestilence, plague, and famine.

"On examining these obstacles to the increase of population, which I have classed under the heads of preventive and positive checks, it will appear that they are all resolvable into moral restraint, vice and misery.

"Of the preventive checks, that which is not followed by irregular gratifications, may properly be termed moral restraint.

"Promiscuous intercourse, unnatural passions, violations of the marriage bed, and improper arts to conceal the consequences of irregular connexions, clearly come under the head of vice.

"Of

"Of the positive checks, those which appear to arise unavoidably from the laws of nature may be called exclusively misery; and those which we obviously bring upon ourselves, such as wars, excesses, and many others which it would be in our power to avoid, are of a mixed nature. They are brought upon us by vice, and their consequences are misery.

"In every country, some of these checks are, with more or less force, in constant operation; yet notwithstanding their general prevalence, there are few states in which there is not a constant effort in the population to increase beyond the means of subsistence. This constant effort as constantly tends to subject the lower classes of society to distress, and to prevent any great permanent amelioration of their condition.

"These effects, in the present state of society, seem to be produced in the following manner :We will suppose the means of subsistence in any country just equal to the easy support of its inhabitants. The constant effort towards population, which is found to act even in the most vicious societies, increases the number of people before the means of subsistence are increased. The food, therefore, which before supported eleven millions, must now be divided among eleven millions and a half. The poor consequently must live much worse, and many of them be reduced to severe distress. The number of labourers, also, being above the proportion of work in the market, the price of labour must tend to fall; while the price of provisions would at the same time tend to rise. The labourer, therefore, must do more work, to earn the same as he did before. During this

season of distress, the discouragements to marriage, and the difficulty of rearing a family are so great, that population is nearly at a stand. In the mean time the cheapness of labour, the plenty of labourers, and the necessity of an increased industry among them, encourage cultivators to employ more labour upon their land; to turn up fresh soil, and to manure and improve more completely what is already in tillage; till ultimately the means of subsistence may become in the same proportion to the population, as at the period from which we set out. The situation of the labourer being then again tolerably comfortable, the restraints to population are in some degree loosened; and, after a short period, the same retrograde and progressive movements, with respect to happiness, are repeated.

"This sort of oscillation will not, probably, be obvious to common view; and it may be difficult even for the most attentive observer to calculate its periods. Yet that, in the generality of old states, some such vibration does exist, though in a much less marked and in a much more irregular manner than I have described it, no reflecting man who considers the subject deeply, can well doubt.

"One principal reason why this oscillation has been less remarked, and less decidedly confirmed by experience than might naturally be expected, is, that the histories of mankind which we possess, are, in general, histories only of the higher classes. We have not many accounts that can be depended upon, of the manners and customs of that part of mankind where these retrograde and progressive movements chiefly take place. A satisfactory history of this kind, of one

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