This cafe will occur whenever a cloud hanging over a building receives fuddenly fuch a charge, by an explosion from another cloud at a distance, as to enable it inftantly to strike into the earth. In this cafe, confidering the inftantaneity (as to fenfe) of the charge, the advantage which is juftly afcribed to the pointed conductor, from its power of gradually drawing off the electric matter, vanishes; as this gradual depletion implies time, which, from the circumftances, is not granted in the prefent cafe. This cafe has been imitated in thofe experiments, which have been made with what is called the interrupted conductor; or a fecond infulated conductor, or fubftitute, placed at such a diftance from a prime conductor, as to receive ftrong fparks from it, or to become fuddenly charged by it. The queftion then is reduced to this ;-whether a vertical cloud may not fuddenly receive fuch a charge, by an explosion from a neighbouring cloud, as may enable it inftantly to ftrike into a pointed conductor, which, it is fuppofed, may invite the ftroke; and yet may be refifted by one that terminates in a ball, fo that it fhall not explode upon it. An appeal has been made to experiment; and both parties allege, that experiment declares in their favour.

The Author attempts to reconcile thefe different refults by the following diftinction. He affirms, that the experiment made with the interrupted or fecond conductor may be fo managed, as that the ball fhall receive the explofion at a greater diftance than the point; or the point be ftruck at a greater diftance than the ball, at the pleasure of the operator; and that this diversity of effect depends on a fingle circumftance; viz. the height of the charge.

If the fecond conductor be placed at fo great a distance from the firft or prime conductor, as to receive from it the fulleft and Arongeft fpark that it is capable of giving; the point placed under the second conductor will be ftruck preferably to the ball, and at a much greater diftance. In a particular cafe, mentioned by the Author, the point is faid to have been ftruck at a distance fix times. greater than that at which the ball would receive a troke. He accounts for this effect, from the greater fufceptibility of pointed bodies to admit the electric fluid, or their giving lefs refiftance to it; combined with their incapacity, in the prefent cafe, of drawing off the electric matter filently.

On the contrary, if the fecond conductor be placed confiderably within the diftance above defcribed, or fo near the firft conductor, as to receive only a weak fpark from it; the ball will be ftruck in preference to the point, or at a much greater diftance; because, in this cafe, the point has an opportunity, on account of the fmallnefs of the charge, of ftcaling away the electricity

electricity filently; which the ball, from its greater refiftance, is incapable of effecting.

Without expreffing any doubts with respect to these experiments of the Author, and this diftinction made by him; we fhall obferve, that whether the preceding cafe frequently occurs in thunder ftorms, or not, it is very remarkable, that though almost all his obfervations relate to this particular cafe; yet we do not find it exemplified, or even noticed, when he afterwards makes a practical application of his doctrine, and enumcrates the various ways in which buildings may become exposed to a ftroke of lightning. And yet, to us, this appears to be the only cafe in which a doubt can reasonably be entertained of the fuperior utility of pointed conductors.

In this enumeration, he obferves firft, that lightning may either accumulate directly (and gradually) over a building;or it may be brought toward it by a fmall cloud, fetching it in feveral fucceffive trips from a large cloud at fome diftance-or laftly, a large electrified cloud may be carried rapidly towards a building by the wind. To none of these three cafes are the experiments made with the fecond conductor applicable; though the Author's principal objections are made to fuch of Mr. Nairne's experiments as relate to this particular cafe. Further, in the two firft of thefe three cafes, he acknowledges the probable efficacy of pointed conductors, in preventing an accumulation of the electric matter, by exhaufting the electrified cloud, fo as to prevent an explofion. On the third cafe, he reasons

thus:

But if, according to the third fuppofition, a cloud of great extent, and highly electrified, fhould be driven with great velocity in fuch a direction, fo as to pafs directly over the sharppointed conductor; there can be no doubt, but that fuch a point, from its fuperior readiness to admit electricity, would take the explosion at a much greater diftance than a rounded end, and, in proportion to the difference of that striking diftance, would do mifchief instead of good.'

Even in this laft cafe, and in all cafes whatever, where time is allowed for the pointed conductor to operate;-and there is no cafe in which time is not allowed, except in that where the vertical cloud fuddenly receives an explosion from another cloud;

we must think that the danger of an explofion must be greatly leffened by the action of the point: for the fame reasons that take place in the two firft of the three cafes; even though we were to admit the truth of the Author's pofition-that a pointed termination will, cæteris paribus, receive an inftantaneous explofion at a much greater diftance than a rounded end. In other words, we think it highly probable, that the difadyantage arifing from the fuppofed rapidity of the cloud's motion may

Dd 3

be

be overbalanced, or more than compenfated for, by the exhaufting power of the point. In Mr. Nairne's three laft experiments above mentioned, when the conductor, variously terminated, was made to fwing, even with great rapidity, under a large electrified conductor; a rounded termination was ftruck, with an equal charge, at a much greater diftance than a pointed extremity. No explofion enfued, when the point paffed at a greater diftance than I inch and ths. A ball ths of an inch in diameter, was ftruck at the diftance of 2 inches and th; and a larger ball of 1 inch and 3ths diameter, received a stroke at the distance of 15 inches *.

TO

Speaking of pointed conductors, at the end of the article, the Author owns, that if by operating upon a quantity of electricity too fmall in itself to do mifchief, they prevent its growing to a great and dangerous quantity; this would, as far as it goes, be a very confiderable advantage.' He adds, however, that pointed conductors afford no protection + where the danger is great and imminent, and only obviate that which is diftant and problematical; and that thefe laft are not the cafes againft which we principally wish to provide.'

Paffing over the cafe above difcuffed, where a very great charge prefents itself, as it were inftantaneously over a pointed conductor; there undoubtedly occur many cafes in which the cloud, A, for inftance, fufpended over a building, receives fucceffive charges from another cloud, B; and that, probably, from the cloud, C; none of which, fingly, are fufficient to enable the cloud, A, to strike into the building: though two or three fuch fucceffive charges might load it fufficiently for that purpose. In this cafe, it cannot be denied, that the continued action of the point on the cloud, A, during the intervals between the explosions, may prevent its ever acquiring fuch a

We should obferve, that the fingular difappearance and re-appearance of the fparks, on gradually removing the fmall conductor from the prime conductor, above noticed at the end of Mr. Nairne's article, occurred likewife in the two first of these cafes: but in the third, where the large ball was ufed, it was ftruck uninterruptedly, till it was removed to the diftance of 16 inches.

+ Dr. Mufgrave has furely expreffed this too lcofely. He cannot mean, that pointed conductors afford no protection to a building, under thefe circumstances; but only that they are more liable to be ftruck, on certain occafions, than blunt conductors. Either a pointed or a blunt conductor may be faid to afford protection to a building, although they should be firuck by the lightning; as little doubt can now be entertained, that either of them will tranfmit it fafely to the earth, provided that they are made of a fufficient fize, and all other circumflances be attended to that are known to facilitate the paffage of the electric matter to the earth.

4

quantity

quantity of electric matter, as to produce an explosion. The point, therefore, will afford protection in this cafe, where the danger is indeed, at firft, fmall and problematical; but would become great and imminent, were the point taken away.

Article 38. On the Ufe of an Amalgam of Zinc, for the Purpofe of electrical Excitation, &c. By Bryant Higgins, M. D.

&c.

This Amalgam, as we have already experienced, greatly exceeds any other that we have tried; and particularly, does not require fuch frequent application, as thofe made with tin, bifmuth, &c. It confifts of zinc, combined with four times its weight of quickfilver. To this amalgam fhould be added, though Dr. Higgins is filent on the fubject, a very small quantity of wax and tallow.-When the cylinder is become foul, the Doctor recommends the use of a piece of the dry fkin of the dog fish, applied to it while it is turned round.

Article 44. New Experiments upon the Leyden Phial, respecting the Termination of Conductors. By Benjamin Willon, Efq; F. R. S.

Thefe experiments were formerly published, at the end of Mr. Wilfon's account of the experiments he made at the Pantheon. They are brought to fhew that, on the discharge of an electrified jar (that is, in the cafe of an inftantaneous explofion, where a point has not time to act upon the electric matter), a point is ftruck at a greater diftance than a ball. The particular refults are given in four tables, figned by Dr. Lind, Mr. Cavallo, and Mr. Wilfon. The contrary refults of Mr. Henly are afcribed by the Author to his having ufed a chain, or an imperfect circuit of communication, inftead of a wire, in his experiments.

Article 47. A ready Way of lighting a Candle, by a very moderate. Electric Spark. By John Ingenhouíz, M. D. F. R. S.

In this paper, Dr. Ingenhoufz teaches the electrical philofopher a very expeditious and commodious way of lighting his taper, by means of an electric fpark from a small charged Leyden vial, that has only eight or ten inches of metallic coating. Round one of the extremities of a bent wire, fome loofe cotton is wrapped, which is rolled in fome fine powder of white or yellow refin. While the other end is held in contact with the outward coating, this end is brought to the knob. The refin takes fire, and communicates its flame to the cotton, and both together burn long enough to light a candle. The fame effect may be produced by the farina Lycopodii, or by dipping the cotton in oil of turpentine; efpecially if a little pin duft be fprinkled upon it: but in both these cafes the charge must be fomewhat greater.

In the night, a large charged jar, prepared in Mr. Cavallo's manner, and which will retain its charge a month, may be kept in readinefs, as a magazine of electric fire; from which the little vial may be inftantly and repeatedly charged, for this purpofe.

Article 48. Electrical Experiments, to explain how far the Pheno mena of the Electrophorus may be accounted for, by Dr. Franklin's Theory of pofitive and negative Electricity, &c. By John Ingenhoufz, M. D. F. R. S.

The theory by which Dr. Ingenhoufz juftly folves the most confiderable phenomena of that curious inftrument, the Electrophorus, was gradually discovered, and fucceffively ascertained, by Mr. Canton, Dr. Franklin, and Meffrs. Wilke and Æpinus. We have formerly explained this doctrine at fome length, in our account of Dr. Priestley's Hiftory of Electricity; [See Monthly Review, vol, xxxvii. October 1767, p. 250, &c.] where we have obferved, "that the electric fluid, when there is a redundancy of it in any body, repels the electric fluid in any other body, within its influence, and drives it into the remote parts, or quite out of the body, if there be any outlet for that purpose; thereby reducing the body to a state contrary to its own, i. e. a negative one." The fame principle is applicable, mutatis mutandis, to a body negatively electrified. Referring the reader to Dr. Ingenhoufz's more minute inveftigation of this inftrument, we fhall give the following fhorter account of its principal properties, corre fponding nevertheless with his explanation, in our own words.

According to the abovementioned principle, fuppofing the refinous plate to have been positively electrified (which cafe admits of a fomewhat eafier illuftration), the excited plate acts upon the electric matter naturally contained in the brass plate, or moveable metallic coating, applied to it; fo as to expel a part of its natural quantity, when an opportunity is given it to efcape from it, in the form of a fpark, at that part where a finger is applied to it. If the brafs plate be now lifted up, by its infulating handle, it is found to be in a negative ftate; and on prefenting a finger to it, another spark appears, which now paffes towards it; and it accordingly receives back its natural share of electric fluid from the finger of the operator. On being replaced on the electrophorus, and again touched, it again parts with a portion of its natural quantity of electric fluid; which is again reftored to it, on lifting it up, and again prefenting the finger to it. This procefs may be repeated for a great length of time, with fcarce any perceptible diminution of the virtue of the refinous plate; which, in fact, does not part with any of its own electric matter to the brafs plate, but, on the contrary, only expels a part of that which naturally belongs to the latter; and which, on lift