This table exhibits the fpecific weights of the various fubftances contained in it, difcovered by fome of the methods already described; and the abfolute weight of a cubic foot of each body is afcertained in avoirdupois ounces by multiplying the number oppofite to it into 1000; as, for example, S. g. of water: S. g. of mercury: 1: 14.019 1000 oz. wt. of a cubical foot of mercury, which is therefore equal to 1000 X 14.109 avoirdupois ounces. There are some uncertainties in this fubject, for fubftances of the fame kind, though denominated by the fame name, may not be precifely fimilar, and fome fmall errors may perhaps be inevitable in phyfical experiments; but they will be inconfiderable if the fcales be nicely adjusted, and the experiments cautiously conducted, fo that the body weighed do not touch the bottom or fides of the vessel, nor rise above the furface of the fluid, nor bubbles of air adhere to it's furface. There is another caufe of uncertainty, for moft fubftances are dilated by heat and contracted by cold, and the dimenfions of the fame body, and confequently it's specific gravity, are different according to the different temperatures of the ambient air; and the altitude of the thermometer ought to be confidered in conftructing a table of fpecific gravities. The different expanfion of bodies in fummer and winter, and confequently their different specific gravities, appear from the experiments of Homberg, and Lifenfchmedites, in his "Difquifitio nova de Ponderibus, &c." from the latter of whom the following table, exhibiting the weight of a cubical inch, Paris meafure, of different fubftances, is taken.

A TABLE

It appears from this table, that the expansions of different fluids are different in the fame changes of the temperature of the air; and it appears-from obfervation, that fubftances not fluid, are also in fimilar circumftances differently dilated; but the weight of given magnitudes, both of fluid and firm bodies, being diminished by heat, and increased by cold, the variation of their fpecific gravities is

lefs

Jefs than if the dimenfions of one of them only had been variable.

OF AIR BALLOONS.

"

The romances of almoft every nation have recorded inftances of perfons being carried through the air, both by the agency of fpirits, and by mechanical inventions; but till the time of the celebrated Lord Bacon, no rational principle appears ever to have been thought of by which this might be accomplished. Before that time, indeed, Friar Bacon had written upon the fubject, and many had been of opinion, that, by means of artificial wings fixed to the arms or legs, a man might fly as well as a bird but thefe opinions were thorougly refuted by Borelli, in his treatife "De motu Animalium," where, from a comparison between the powers of the mufcles which move the wings of a bird, and thofe which move the arms of a man, he demonftrates that the latter are utterly infufficient to ftrike the air with fuch force as to raise him from the ground. It cannot be denied, however, that wings of this kind, if properly conftructed and dextroufly managed, might be fufficient to break the fall of a human body from an high place, fo that fome adventurers in this way might poffibly come off with fafety; though by far the greatest number of those who have rafhly adopted fuch fchemes, have either loft their lives or limbs in the attempt.

In the year 1672, Bishop Wilkins published a treatise, entitled, "The Discovery of the New World," in which he mentions, though in a very indiftinct and confufed manner, the true principle on which the air is navigable; quoting from Albertus de Saxonia and Francis Mendoca, "that the

air is in fome part of it navigable; and upon this ftatic principle, any brafs or iron veffel (fuppofe a kettle), whofe fubftance is much heavier than that of water, yet being filled with the lighter air, it will swim upon it and not fink. So, fuppofe a cup or wooden veffel upon the outward borders of this elementary air, the capacity of it being filled with fire, or rather etherial air, it muft neceffarily, upon the fame ground, remain swimming there, and of itfelf can no more fall than an empty fhip can fink." This idea, however, he did not by any means pursue, but refted his hopes entirely upon mechanical motions, to be accomplished by the mere ftrength of a man, or by fprings, &c. and which have been demonftrated incapable of anfwering any useful purpose.

The only person who brought this fcheme of flying to any rational principle, was the jefuit Francis Lang, cotemporary with Bishop Wilkins; he being acquainted with the real weight of the atmosphere, juftly concluded, that if a globular veffel was exhaufted of air, it would weigh lefs than before; and confidered that the folid contents of veffels increase in much greater proportion than their furfaces; he fuppofed that a metalline veffel might be made fo large, that when emptied of it's air, it would be able not only to raise itself in the atmosphere, but to carry up paffengers along with it; and he made a number of calculations neceffary for putting the project in execution. But though the theory was here unexceptionable, the means proposed were certainly infufficient to accomplish the end; for a veffel of copper, made fo thin as was neceffary to make it float in the atmofphere, would be utterly unable to refift the external preffure, which being demonftrated by thofe fkilled in mechanics, no attempt was made on that principle,

In the year 1766, Mr. Henry Cavendish afcertained the weight and other properties of inflammable air, determining it to be at leaft feven times. lighter than common air. Soon after which it occurred to Dr. Black, that perhaps a thin bag, filled with inflammable air, might be buoyed up by the common atmosphere, and he thought of having the allantois of a calf prepared for this purpose; but his other avocations prevented him from profecuting the experiment. The fame thought occurred fome years afterwards to Mr. Cavallo; and he has the honour of being the firft who made experiments on the fubject. He first tried bladders; but the thinneft of thefe, however well fcraped and prepared, were found too heavy. He then tried Chinese paper; but that proved fo permeable, that the vapour paffed through it like water through a fieve. His experiments, therefore, made in the year 1782, proceeded no farther than blowing up foap-bubbles with inflammable air; which afcended rapidly to the cieling, and broke against it.

But while the difcovery of the art of areostation feemed thus on the point of being made in Britain, it was all at once announced in France, and that from a quarter whence nothing of the kind was to have been expected. Two brothers, Stephen and John Montgolfier, natives of Annonay, and mafters of a confiderable paper manufactory there, had turned their thoughts towards this project as early as the middle of the year 1782. The idea was firft fuggefted by the natural afcent of the fmoke and clouds in the atmosphere; and their defign was to form an artificial cloud, by inclofing the fmoke in a bag, and making it carry up the covering along with it.

Towards the middle of November, that year, the experiment was made at Avignon, with a fine filk bag, of a parallelopiped fhape. By applying burning