« AnkstesnisTęsti »
hood was miferably afflicted by the noxious exhalation which it fent forth.
There are fome forts of earth from which allum is made, which abound fo much with the pyrites, that the proprietors of the works are forced to keep them conftantly well watered, in order to prevent their taking fire.-But it would be useless to pursue this fubject further; we have adduced pooof fufficient, that nature furnishes materials, which, under certain circumftances, may become the occafion of fubterraneous fires. The requifite circumftances are a proper quantity of the materials, a proper portion of water to moisten them, and perhaps a communication with the air may be neceffary. A fmall quantity of the pyrites is fufficient to kindle a fire; water is almost every where found in fuch great plenty below the furface of the earth, that it conftitutes one of the greatest impediments to our finking pits to any great depth; and air, if it should be thought abfolutely neceffary to the fpontaneous firing of the pyrites, may be conceived either to accompany the water in its dripping, or to defcend into the innermost parts of the earth through the fiffures which are found upon its furface.
Thefe inftances of fpontaneous fires arifing from minerals, naturally remind us of the danger to which we are perpetually exposed by fuch combinations and effects. In fact, they often have occurred, veftiges of fires are every where around us; they often are occurring, and I think I may venture to say, they would occur more frequently, were they not reftrained.. There feems little occafion to doubt the poffibility of that general
neral conflagration of which we are fore rned; there are fo
LADIES and GENTLEMEN,
E have treated on those principles, which, from their combination with, and formation of, all fenfible objects on this terrestrial planet, are denominated ELEMENTS, Air, Water, Earth, Fire, whofe influences are extenfive, intimate, and indispensable; we trace them with certainty in their effects, and entertain no doubt of their powers, though many difficulties impede our knowledge of their modes of action, or the fprings and movements, which, by their means, produce fuch effects as daily occur around us. mains now, that we hint a few remarks on other principles not lefs interesting, though less frequently the subject of our attention.
We have found reafon to fuppofe, that of the four elements, three were fluids; and indeed fluids, by their mutability, feem well adapted for general distribution and use; as their equilibrium, or state of reft, is without difficulty disturbed, their courfe is more easily directed, and when the impulfe which actuates them ceases, they more readily return to that quiescent state which is natural to them. It muft, however, be admitted, that by this very circumstance they are greatly conXII. K cealed ३
cealed from our infpection, and our acquaintance with them is confequently very much impeded.
Befide the fluids we have mentioned, I wish to introduce a few words on that, which is ufually termed the MAGNETIC fluid, because first indicated by its connection with the magnet, or loadstone, though now difcovered to affect iron and fteel univerfally, and to abound wherever we have carried our refearches.
The natural magnet, or loadftone, is a kind of ore of iron, which has been in all ages famous for attracting pieces of iron from a small distance, and by fuch attraction fuftaining pro portionate weights of that metal. THALES, amazed at so constant an effect, thought this ftone had a foul. This most obvious property of the loadftone, of itself fufficiently embarraffing and inexplicable, yet is but the fign of a power infinitely more astonishing; in fact, this fubject is a striking proof that nature has many fecrets, and that philosophy, if contented with prefent knowledge, foregoes most valuable and interesting discoveries, toward which, perhaps, the previous steps are already trodden. About the close of the eleventh, or beginning of the twelfth century, it was observed, that when the loadftone enjoyed free motion (as when swimming on a piece of cork in a bafon of water), though its direc tion might be disturbed by the most rapid whirl, yet it would recover the position it had quitted; always pointing its fame fide to the North.
Experiment difcovered, that the qualities of the magnet might be communicated from the natural ore to iron in gene
ral; and that when fmall bars of fteel (called needles) were brought into contact with it, and properly rubbed upon it, they acquired these virtues in very confiderable degrees. This operation was called touching; and hence originated the mariner's compafs; an invention, by which the intercourse of mankind is inconceivably promoted, which, by giving scientific fupport to courage and fkill, has done more at a stroke toward rendering all nations as one family, than all preceding ages, and all the endeavours of philosophy or learning could accomplish. If it is afked, by what means we furround the globe? how we dare venture on unknown coafts, and how we discover our fituation, wherever placed on the face of the earth? We answer these, and a thousand other questions of fimilar import, by a reference to the compafs. If we fpeak of immenfe continents difcovered-it was by the aid of the compass; if we mention voyages from pole to pole-they are performed by direction of the compafs: the life, the fecurity, the perfection of navigation,—is the compass.
When the prodigious utility of the compafs became generally known, nothing could be more natural, than to enquire on what principle this conftancy of the needle depended, and whether iron might not be magnetic in other ftates than as ore. The result of fuch enquiries has been a conviction, that the principle of magnetism is very generally diftributed, and that iron either poffeffes or acquires it, long after it has been separated from the mineral, and wrought into household utenfils. Scarce a poker, or pair of tongs, which are not constantly in ufe, but which (as in parlours) are fuffered to remain