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ter in the upper regions. This, perhaps, is the caufe, why the air is fo clear and transparent in warm climates.

"By making fome obfervations on the falling of rain, we fhall have other proofs, that the electric matter is the great caufe by which vapour is fupported in the atmosphere. Here I muft obferve a fact, well known to all prefent, that bodies electrified, by the fame electric power (no matter whether pofitive or negative) repel each other; and, when electrified by the different powers, that is, the one plus and the other minus, attract each other: on coming into contact, an equilibrium is restored, and neither of them will fhew any figns of electricity.

"From this it follows: if two clouds are electrified by the fame power, they will repel each other, and the vapour be fufpended in both; but, when one is pofitive and the other negative, they will attract each other, and restore an equilibrium. The electric power, by which the vapour was fufpended, being now destroyed by the mutual action of the clouds on each other, the particles of water will have an opportunity of running together into each other, and, as they augment in fize, will gain a greater degree of gravity, defcending in fmall rain, or a heavy fhower, according to circumstances.

"A cloud, highly electrified, paffing over a high building or mountain, may be attracted by, and be deprived of its electricity, with out or with a violent explosion of thunder. If the cloud is electrified plus, the fire will defcend from the cloud to the mountain; but, if it be electrified minus, the fire will afçend from the mountain to the cloud. In both cafes, the effect is

the fame, and generally, heavy rain immediately, or foon after, follows: this is well known to the inhabitants of, and travellers among, mountains.

"From this we can cafily ac count, why thunder-showers are often partial, falling near, or among mountains, and the rain in fuch quantities, as to occafion rivers to be overflowed; whilft, at the di ftance of a few miles, the ground continues parched up with drought, and the roads covered with duit.

"It often happens, that one clap of thunder is not fufficient to produce rain from a cloud, nor even a fecond: in fhort, the claps must be repeated, till an equilibrium is reftored, and then the rain muft, of confequence, fall. Sometimes we may have violent thunder and light. ning without rain, and the black appearance of the heavens may be changed to a clear tranfparent sky, efpecially in warm weather. To account for this, it must be remem bered, as I lately faid, that one or more claps of thunder are not always fufficient to produce rain from the clouds: fo, if an equilibrium be not restored, little or no rain will fall, and in a fhort time the electric matter, paffing from the earth to the clouds, or the fuperabundant quantity in the air, will electrify thofe black clouds, by which means the particles of vapour will be expanded, raised higher, and the air become clear. Clouds may be melted away, even when we are looking at them, by another caufe, that is, by the heat of the fun. We know, that tranfparent bodies are not heated by the fun, but opaque ones are; the clouds being opaque bodies, are warmed by the rays of the fun thining on them, and any additional quantity of heat will rarify the va pour, and occafion its expanding in



the air, which will foon become transparent. When vapour is made to expand more than it would otherwife do, a certain quantity of abfolute heat is neceflary to keep it in the form of vapour; therefore, when the receiver of an air-pump is exhaufting, it appears muddy, and a number of drops are found within it; the moisture contained in the air, in the form of vapour, being made to occupy a greater fpace than what is natural to it, and receiving no addition of heat, a part of it is condensed.

If, therefore, the air is fuddenly rarified, a few drops of rain will defcend, as may often be ob

ferved in the fummer feason.

"I have repeatedly obferved, efpecially during the fummer, when the wind is at north-east, that the weather is, in general, cold and dry, with a clear atmosphere. Should the wind fuddenly change to fouth-west, in a few hours, black clouds begin to gather, vegetables look fickly, and droop their leaves; and, foon after, comes on a violent ftorm of thunder, with heavy rain. "This change, I imagine, is not fo much owing to the fouth-west wind bringing rain, as to the atmofphere's being changed from an electric ftate, capable of fufpending vapour, to a ftate of parting with its moisture. As foon as the ftorm is going off, vegetables revive from their languid ftate, and the air recovers its ufual afpect. From this we may conclude, that no inftrument can be made to afcertain the quantity of moisture in the air: all that is, or ought to be expected from a hygrometer, is to fhew, whether the air be in a state to retain or part with its moisture. In apparent dry weather it may point to rain; and when it rains, it may

point to fair. For this reafon, the ftones of halls, and fmooth fubftances, are often bedewed with wet, in dry warm weather (that is, the airs in a state to part with its moifture), and, vite verfa, they will dry in the time of rain.

"Left this paper should exceed the common limits of time in reading, I fhall pafs over thofe obfervations, which might be made on fogs or mifts; a few excepted, which I fhall here fubjoin.

"Fogs are produced by two caufes as different as their effects are oppofite. A fog may be produced by a precipitation of rain, in very fmall particles, like a cloud floating on the furface of the earth. In this cafe the air is moist and damp, and never fails to wet a traveller's cloaths; the ftones of the street, painted doors, and hard, cool, fmooth bodies, are generally covered with moisture, which often runs in large drops: this, I dare fay, has been obferved by every perfon. Secondly, a fog may be produced by the abforption of moisture, when the air is too dry, and differs from the other juft defcribed; for it will not impart any of its moisture even to dry bodies; no damp is to be met with on ftones, polished marble, &c. This fact is well known to the inhabitants on the fea-coast of Fifefhire, who, during their fummer months, have frequent opportunities of obferving a fog in the afternoon, driving up the Firth of Forth, with a drying eaft wind, which of ten blafts the trees and young vegetables, and, therefore, in a small degree, refembles the Harmattan in drying up the ground, and robbing vegetables of their moisture.

"I fhall now conclude with a fhort fummary of the whole.

"1. That heat is the great caufe,


by which water is converted into vapour, which is condenfed by cold.

46 2. That electricity renders vapour fpecifically lighter, and adds to its abfolute heat, repelling its particles; which particles would be condenfed by cold: and that electricity is the great agent by which vapour afcends to the upper regions.

"3. That when the electric power, by which vapour is fufpended in the atmosphere, is deftroyed, a heavy miit, imall rain, or thunder-fhowers, will be the contequence. Had the advocates for the doctrine of folution made heat and electricity the folvents, their theory would have been lefs exceptionable."




prey, which may be confidered as its tentacula, about an inch and a quarter.

"The body of the animal is at tached to its fhell, for about three. quarters of an inch in length, at the anterior part where the two cones arife, by means of two cartilaginous fubftances, with one fide adapted to the body of the animal, the other to the internal furface of the fhell: the rest of the body is unattached, of a darkish white colour, about half an inch broad, a little flattened, and rather narrower towards the tail. The mufcular fibres upon its back are transverse; thofe on the belly longitudinal, making a band the whole length of the body, on the edge of which the tranfverfe fibres running across the back terminate.

"The two cartilaginous fubftances by which the animal adheres to its fhell, are placed one on each fide of the body, and are joined together upon the back of the animal at their posterior edges: they are about three quarters of an inch long, are very narrow at their anterior end, becoming broader as they go backwards; and at their poflerior end they are the whole breadth of the body of the animal. Upon their external furface there are fix tranfverfe ridges, or narrow folds; and along their external edges, at the end or termination of each ridge, is a little eminence refembling the point of a hair pencil, fo that on each fide of the animal there are fix of these little projecting ftuds, for the purpofe of adhering to the fides of the Thell in which the animal is inclofed. The internal furfaces of thefe cartilages are firmly attached to the body of the animal, in their middle part, by a kind of band or liga

ment; but the upper and lower ends are lying loofe.

"From the end of the body, between the two upper ends of thefe cartilages, arife what I fuppofe to be the tentacula, confitting of two cones, each having a spiral membrane twining round it: they are clofe to each other at their bafes, and diverge as they rife up, being about an inch and a quarter in length, and nearly one-fixth of an inch in thickness at their base, and gradually diminishing till they terminate in points. The membranes which twine round these cones alfo take their origin from the body of the animal, and make five fpiral turns and a half round each, being loft in the points of the cones; they are loote from the cone at the lowest fpiral turn which they make, and are nearly half an inch in breadth; they are exceedingly delicate, and have at small diftances fibres running across them from their attachment at the stem to the loofe edge, which gives them a ribbed appearance. Thefe fibres are continued about one-tenth of an inch beyond the membrane, having their edges finely ferrated, like the tentacula of the Actinia found in Barbadoes: thefe tentacula fhorten as the fpiral turns become smaller, and are entirely loft in that part of the membrane which terminates in the point of the cone.

"Behind the origin of thefe cones arifes a fmall fhell, which, for one fixth of an inch from its attachment to the animal, is very flender: it is about three-quarters of an inch in length, becoming confiderably broader at the other end, which is flat, and about onethird of an inch broad; the flattened extremity is covered with a kind of hair, and has rifing out of

gularity utterly irreconcilable to the common hypothefis: at least, I am acquainted with no plausible answer to this objection.

"Should any one reply, "that alphabetical characters may have been in existence many ages prior to the date of thefe fpecimens in the Scriptures, but that the more ancient memorials, in which they were exhibited, have perished by the defolations of ignorance and the viciffitudes of time:" I must demur at an argument that advances no premifes of fufficient validity to authenticate this conclufion. For, 1. It is mere affirina tion, without the leaft fhadow of historical teftimony to give it countenance. 2. To wave the authority of the Jewish fcriptures upon this point (which, however, I muft beg leave to obferve, is corroborated by abundant evidence from philofophy and experience, as well as hiftory), that fimplicity of manners, predominant in the early ages, fo obfervable in the accounts delivered down by facred and profane hiftorians; the confeffed mediocrity of their intellectual acquirements, and the confined intercourfe of nations with each other, which would render such an expedient lefs neceffary, and therefore less likely to be discovered: all thefe confiderations seem to argue with no little cogency, that fo complex, fo curious, fo wonderful, fo confummate a devife as that of alphabetical writing, could hardly be first detected by a race of men, whofe wants were few, whofe advantages were circumfcribed, and whose ideas were commenfurate to their fituation. This pofition, therefore, conjectural as it is, and unfubftantial, feems unwor thy of farther animadverfion.

II. Ifalphabetical writing were a human invention, the natural re

fult of ingenuity and experience, might we not expect that different nations would have fallen upon the fame expedient, independently of each other, during the compass of fo many ages: when the faculties of the mind are equally capable at all times, and in every corner of the univerfe, and when the habits of life and modes of thought inevit ably bear fo great a resemblance to each other in fimilar stages of fociety? This, I fay, were but a reafonable expectation: which, however, correfponds not to the event. For alphabetical writing, as now practifed by every people in the univerfe, may be referred to one common original, If this propofi tion can be proved, the argument from fucceffive derivation, without a fingle inftance of independent dif covery, must be allowed to amount to the very highest degree of probability in my favour: and the common fuppofition will appear perfectly gratuitous, with the incumbrance alfo of this great paradox: "You tell us, I might say, of an invention, which is the regular confequence of refinement in fociety, nothing more than a gra dual advancement from what is plain to what is complex; by a fimilar procefs, purfued by the mind in all its exertions for improve ment: and yet we can perceive no reafon to conclude, that any community but one, and that in mo wife diftinguished by any vaft fuperiority of inventive genius, or the improvements introduced by them into common life, ever compaffed this discovery; though the human powers have been uniformly the fame, and the conduct of fociety has been greatly fimilar in different nations at different periods of time."

"Let us confider then, how the evidence ftands in this cafe: only


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