in a little time the air becomes vitreously electrified, and then both it and the cloud act as one body. The furface of the earth then begins to be electrified, and the powers therein to be separated; and a continual effort is made by the contrary electricities to unite between the earth and the cloud. If thofe caufes which first produced the electricity ftill act, the power becomes inconceivably great, and the flashes in uniting will tear every thing to pieces that refift their paffage.

Mr. Read juftly obferves, that a portion of the earth may be highly electrified, and yet we may be infenfible thereof, because we are involved therein; for where all things are equally involved in an electrical atmosphere, there can be no visible signs of the prefence of the electric matter. Thus if two or more perfons be electrified, while ftanding on the fame infulation, they fhew no figns to each other of being electrified.* Whatever be a perfon's fituation, whether in the houfe or open field, he is liable to be involved in an electric charge, whether it be stationary, or moving with the clouds. Mr. Read found himself fo involved once in Hyde Park; the atmosphere had a menacing appearance with a heavy black cloud at no great diftance; on taking his pocket electrometer out of it's cafe, and holding it in his hand, it inftantly diverged near one inch. It is not probable, that the restoration of the equilibrium, or returning froke, as it is often called, will hurt any one, unless they are in the direct path of the flash.

I have already obferved, that it is probable that the operations of the electrical matter are moft univerfal and important in it's latent and united state; and that whenever by feparation it becomes visible, there is then a general ftrefs Bb4 throughout

* Read, p. 61. Ibid. p. 68.

throughout the greater part of our system, to reftore the equilibrium; and that this ftrefs is greater in proportion to the quantity feparated; that this feparation in many inftances is fpontaneous; and that as this fluid is univerfally diffeminated, there is no occafion to confider the appearance of electricity in vapour, &c. as the means whereby this fluid is conveyed to the clouds.

From M. de Luc's obfervations, it would hence appear, that lightning often arifes from fud'den production of a great quantity of the electrical fluid, that which is then manifefted, not being apparent as electricity, but juft before we perceive it's effects. This is further confirmed by his obfervations when on mountains, where he had often opportunities of viewing thefe phenomena. Thus in a storm on the Buet, (one of the Alps) while the air was perfedly transparent and dry, (the last circumftance being determined by the hygrometer) clouds began to form in different parts; thefe, when thickened and, united, embraced the fummit of the Buet, and fupported themselves against Mount Blanc, and the fummits of the neighbouring mountains. Mr. de Luc and his companions were overwhelmed with rain; there was alfo a vaft deal of lightning, which was often violent, and lafted for a confiderable time. Mr. de Sauffure has alfo given inftances where the clouds formed a conducting communication with the ground, and yet the lightning continued without interruption:

From thefe phenomena, air perfectly tranfparent and dry, containing neither the vapours of which the cloud is formed, nor the electric fluid, but only the ingredients proper to give them birth; he infers, that by fome unknown caufe, clouds of a certain kind are formed fpontaneously, and dur

ing the progrefs of their formation, the electricity is produced in great abundance, exploding every time it is thus formed; and that before this, the electric fluid no more exifted in that state, than the aerial fluids, which are difengaged from gunpowder, existed as fuch before the gunpowder was exploded. I need scarce observe to you, how much Mr. Eeles's theory is confirmed by this account of Mr. de Luc.

You may gain fome idea of the prodigious quantity of the electric fluid, that is fometimes manifefted, and paffing between the clouds and the earth, by an inftance or two with which we are furnished by Mr. de Luc. Thus a cloud was obferved at the top of the mountains of Turin: it was formed of a mafs, whofe obfcurity rendered it terrific, producing in thofe places, over which it was fituated, night at noon day; this mafs was ploughed as it were by lightning, which was foon after followed by a grumbling kind of thunder; there fell fo prodigious a quantity of water and ice from this cloud, that the country was ravaged by the torrents, the hedges were beat down, and the ditches half filled with hail. Erfurt, a fmall city in Germany, was ftruck in one night in forty-two different places; feven perfons were killed, three houfes were fet on fire, but quenched by the rain, which came down in torrents. Now where shall we find, on the vapour theory, known humidity in any ftrata of transparent air, fufficient to explain the formation of fuch clouds, and the torrents of rain which was difcharged from them?

OF CONDUCTING RODS.

We are now prepared to confider the advantage of conducting rods. You know that the elecrical fluid is always impelled to thofe places where

an

an exchange of powers can be moft eafily made, or where the union of the two powers is leaft refifted. If, then, there happens in any of the preceding inftances to be a houfe furnished with a conducting rod, directly between that part of the cloud, and that part of the earth, where there is the greatest effort for reftoring the equilibrium, the conductor will be ftruck, and will probably prevent the building from receiving any injury. If there be no conductor, the lightning will for the foregoing reafons pafs at the fame place, but the building will probably be damaged, because the materials refift the paffage of the electrical powers.

OF POINTED AND KNOBBED CONDUCTORS.

A great difpute has been carried on among electricians concerning the termination of conducting rods, for preferving buildings from lightning; fome warmly contending that they fhould be terminated by knobs or balls; others as ftrenuoufly contending, that they fhould be pointed.

Ever fince the identity of electricity and lightning has been proved, conductors of fome kind have been generally allowed to be neceffary for the fafety of buildings in thunder ftorms, as they afford a ready paffage for the union of the contrary electricities. Electricians feem to have forgot that neither lightning nor electricity ever ftrike a body, merely for the fake of the body, but because that body is a means of reftoring the disturbed equilibrium.

When a quantity of electricity is excited by means of an electric machine, a body communicating with the earth, will receive a ftrong fpark-from the prime conductor; it receives this spark, not be cause it is capable of containing all the electricity

of the cylinder and conductor, but becaufe the natural fituation of the fluid being difturbed by the motion of the machine, the natural powers make an effort to restore the equilibrium, No fooner, then, is a conducting body, communicating with the earth, prefented to the prime conductor, than the whole effort of the electricity is directed against that body; not merely because it is a conductor, but becaufe it affords a place, by which the natural powers can more readily unite, and which they would do by other means, though that body were not to be prefented. That this is the cafe, we may eafily fee, by prefenting the fame conducting fubftance in an infulated ftate to the prime conductor of the machine, when we fhall find only a small fpark will be produced. In like manner, when lightning ftrikes a tree, a houfe, or a conducting rod, it is not because these objects are high, but because they are fituated in that place, where, from a variety of caufes, the impetus of the two powers can be leffened by uniting with each other.

From hence you will perceive the fallacy of that kind of reafoning, which is generally employed concerning the ufe of thunder rods.

Because a point prefented to an electrified body in our experiments, draws off the electricity in a filent manner, Dr. Franklin and his followers have concluded, that a pointed conductor will do the fame thing to a thunder cloud, and thus prevent any kind of danger from a stroke of lightning.

But for this very reason, Mr. Wilson and his party have determined, that the ufe of pointed conductors is utterly unfafe; they justly confider the Franklinian idea of exhaufting the clouds of their electricity, to be not lefs abfurd, than it would be, to clear away an inundation with a shovel, or exhauft the atmosphere with an air-pump.

The