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twilight of an hour; at the poles, twilight prolongs all-cheering day, here is a twilight of many hours; every glimpse of light is valuable here, let the poles enjoy it by refraction.

We fhall now confider the atmosphere, as contributing very greatly to the enjoyment of other fenfes of human life its effect indeed feems not very powerful in the articles of tafting or feeling, or at least, it is not very evident in what manner it contributes to thofe fenfations; but the atmosphere is exclufively the medium of found, and if we were deftitute of such a furrounding fluid, we should neither enjoy the cheerful voice of melody, nor the focial endearments of friendship conversation, inftructive or delightful, would be unknown, joyful expreffion, or anxious folicitation would never thrill in the awakened ear; no nor the tender accents of lifping infancy, or the nobleft inductions of manly reafon.

Sound is an idea excited in the mind by means of the auditory nerves, which receiving impreffions from the external air, communicate corresponding impreffions to the brain. In the air itself, found is propagated or diftributed from place to place, by means of certain tremors or vibrations which originate. in the fonorous body. An inftance may explain this idea; observe the vibrations of a large bell ftruck by its clapper or a hamthefe produce correfpondent vibrations in the immediately furrounding air; from this furrounding air they are tranfinitted to the air around that, through this to other air more diftant, and fo on, ever decreasing in ftrength of tone, as increasing in

mer;

diftance,

distance: much as the circling waves which ruffle the finooth surface of a standing pool, when you throw a pebble into it, are first strong and clear, but gradually enfeebled as they enlarge in dimenfions, they decrease in ftrength, till their impulfe is too fmall to move the quiefcent fluid.

Water is capable of receiving these motions on feveral parts of its furface at once, and the circular waves in their courfes will crofs, and, as it were, go through each other, notwithstanding their reciprocal refiftance: thus, founds from all parts may reach the ear at the fame time, and, according to their force, they suggest the idea of distance. The idea of direction (i. e. from what quarter they come) feems to be acquired by the mind in confequence of much attention, and to be learnt by frequent remark and correction.

It is a mighty pretty experiment, performed by putting a lit tle bell under the exhaufted receiver of an air-pump, in which fituation it makes no found; I mean, that however the clapper, by ftriking the fides of the bell, may caufe them to vibrate, yet the space around the bell being void of air to receive those vibrations, they are totally impotent, for want of a medium to tranfmit them. In condenfed air found is proportionately ftrengthened. We fhall have occafion to recur to further principles on this article when investigating the fenfe of hearing; I therefore shall now only remark, that the atmos phere transmits found through very great distances, but if any impediment be interpofed, it returns the agitated waves from fuch impediment; and this return of found we call an echo.

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The distance to which found may be carried by the atmofphere, may well be fuppofed proportionate to the magnitude of the fonorous body, to the intensity and force of the stroke acting on such body, and to the state of the atmosphere itself, whether clear and pure, or vaporated; to which we must add, the course and strength of the wind; since that may either accelerate, or retard the wave of found, either prolong or shorten it. Sound has been frequently heard fifty, or fixty, or eighty miles; but Dr. Hearn, phyfician to the king of Sweden, tells us, that the found of the bombardment of Holmia, A. D. 1658, was heard an hundred and eighty miles; and fome have thought, the guns at the action of La Hogue were heard two hundred miles.

The immenfe variety of proceffes conftantly carrying on in the atmosphere, the numerous objects which occur to divert found from its courfe, and to impede its progrefs, may very much leffen its effects from what they would be in a medium ftill, pure, and free. This feems confirmed, by the strength of the voice in caverns and vaults, where the air is at reft, and by the distance at which the voice may be heard on a smooth river: perhaps I may add, the inftance of a countryman, who being employed to dig a deep pit, was frightened from his work by dreadful noifes! which proved to be thofe arifing from the trotting of a flock of sheep at two miles distance, but tranfmitted to him by fome fubterraneous conveyance.

The principles of echoes are not unlike thofe of reflected light; they are reverberations or returns of found from fome

body

body against which, what I may term the rays of found, strike, and unable to pafs through it, are beaten back, and returned. This returning may arise from many kinds of objects, from a wall, from a hill, or if other impediments to the progrefs of found be wanting, the atmosphere furnishes clouds, which often create echoes to great founds. There are many very curious echoes mentioned by authors; fome natural, some artificial; in fome, the found is repeated twenty, thirty, even fifty times, fome repeat fyllables, and fome words, or fhort fentences; this is easily accounted for on the principles just hinted,

We shall turn our thoughts for a moment to the uses of the atmosphere as they regard the sense of SMELLING. We may confider a flower, or other odoriferous body, as a centre from whence are diverged thousands of powerful and active atoms, which, when they meet our olfactory nerves, excite a fmell, of whatever nature it may, pleafant or foetid, agreeable or offenfive. These atoms, or particles, are received into the atmosphere, where they float, and whereby they are distributed far and wide, Perhaps, I may illuftrate the share of the atmosphere in this bufinefs, by referring you to a bason of clear water, in which if you drop a little coloured liquor, the water, foon becomes tinged, and the colour of the liquor fpreads through the whole; attenuated, weakened, and thinned; which effect, no doubt, takes place on a larger fcale in the atmosphere with regard to odoriferous particles; and very much to our benefit, fince we well know, that many fmells are too ftrong to be endured without inconvenience; that some are fatal though fragrant, because undiluted (witness

the

the pea-bloffom, damp of mines) and that most are unhealthy if too conftantly reforted to. The tea-brokers in London (who judge of tea by fmelling it ftrongly) are greatly liable to the palfy; and the perpetual use of perfumes, among the women of Chili, produces an internal disease, as dreadful as incurable. But when a free circulation imbibes fragrance, or when fragrance is in its nature moderate, then it conduces to health and cheerfulness, refreshes the fpirits, and exhilarates the fenfations; fuch is the fmell of morning and evening, arising from vegetation; fuch is new mown hay; and many others, which are happily no ftrangers nor rarities among us.

See from one fimple covering furrounding our habitation, what various fervices are bestowed on the children of men; what various fervices which come to our knowledge; but could we see the magazines of fnow, the treasures of hail, the repofitories of froft, the refervoir of the drops of the dew; could we trace the fprings and circulations of the electric fire, of the magnetic fluid (at which we can but guess); could we defcend to the minutia of thofe proceffes conftantly in motion, whereby vegetation is advanced, fertility increased, life prolonged by refpiration, and many other beneficial effects produced, we might juftly lay our fingers on our lips in adoring filence, and mufe that praise to which expreffion is inadequate.

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