cuit, one against the other. For, not insulating the jar, but setting it on the table, which gave the circuit and the bodies contiguous to it an advantage for contracting positive electricity by the discharge; but, at the same time, making an interruption in the circuit (by introducing part of the table into it, which tended to give them negative electricity); he could easily manage it so, that the circuit contracted neither the one nor the other; and yet, as in the former case, the lateral explosion was as considerable as ever. The balls never separated.

To vary the experiment, he placed an insulated brass ball, 2 inches in diameter, round and smooth, so as not easily to part with any electricity it had got, in the place of the rod that supported the pith balls; and having found a situ ation in which no electricity was communicated to the circuit, he observed that none was communicated to it, though, to all appearance, it received a spark of about of an inch in length. At least, if it had contracted any, it was so little, as to make it very problematical; whether a pith ball, or a fine thread, was moved by it, or not: whereas, when he gave it the smallest sensible spark in any other manner, it would attract those light bodies for a long time together. The interruption of the circuit made use of in this experiment, was not by means of any part of the table, but only about a yard of brass chain introduced into it, and disposed between the inside of the jar and that part of the circuit, near which the insulated ball was placed. N. B. The ball must not be placed near the jar itself; for, in that situation, he found, that, though it was very smooth, and perfectly spherical, yet it could not be placed very near the outside of the jar standing on the table, without contracting negative electricity, in a very small space of time.

These experiments threw him back into his former state of perplexity, with respect to the lateral spark; since, when the two electricities of the circuit were exactly balanced, it was very little diminished, and yet the body that received it was not in the least sensibly electrified. But, on reflection, he concluded, that this lateral spark must be of the nature of an explosion, and consequently, that an electric spark must enter, and pass out again, within so short a space of time, as not to be distinguished, and leave no sensible effect whatever: for though, in this case, part of the electric matter natural to the body must be repelled, to make room for the foreign electricity, its restoration to its natural state was so quick, that no other motion could correspond to it. This hypothesis is favoured by the observation, that it is the very same thing, whether a body be introduced into a circuit. or placed near it, with respect to contracting electricity; that is, whether the electric charge enter the body at one place, and go out at another, or whether it be received and emitted at the same place. This lateral explosion is an effect similar to a partial circuit, in which, part of the electric matter that forms the charge in an explosion, goes one way, while the rest of the

charge goes another; the only difference is, that this detached part of the charge leaves the common track, and returns to it again, in the very same place.

Several remarkable partial circuits occurred in the course of his experiments before, particularly one, mentioned in the History of Electricity, p. 692, in which, part only of the explosion passed in the shortest way, while another part of it took a circuit, consisting of the same materials, 30 times as long; and another, mentioned, p. 691, where one circuit was made through a thick rod of metal, and another, at the same time, through the open air.

That there is an admission and an explosion of the electric matter, in this lateral explosion, seems evident, from this circumstance, that it is far more considerable when the body that receives it is large, than when it is small. In the former case, there is room for the electric matter, natural to the body, to retire, on the admission of the foreign electricity belonging to the charge, whereas, in the latter case, there is not room for it. When he placed a small brass ball, of about a quarter of an inch in diameter, near the circuit, he could not perceive that it was at all affected by any lateral explosion; and the spark was very inconsiderable, when he placed a needle, about 2 inches in length, to receive it; but when he connected the large tube above mentioned, by means of a pretty thick iron wire, to any body whatever, placed in the neighbourhood of the circuit, he had (with a jar of only half a square foot of coating glass) made the lateral explosion, an inch or more in length, consisting of a very full and bright spark of electric fire. Insulated bodies, of about 8 or 9 feet in length, seem to admit as large a lateral explosion as any body whatever is capable of: for, connecting them with the ground, by means of the best conductors (which gave the electric matter in the bodies, the freest recess possible) he could never make this explosion much more considerable, using the same jar, and all other circumstances the same.

It is a manifest advantage in these experiments, that the lateral explosion be not taken from the coating of the jar itself, or from any part of the circuit, very near to it. He found that, cæteris paribus, it is the most considerable when taken at the extremity of a brass rod, of one foot, or a foot and a half long, the other end of which is contiguous to the jar. It is analogous to this, that the longest spark is taken, not from the body of the prime conductor itself, but at the extremity of a long rod inserted into it. The electric matter seems to acquire a kind of impetus by the length of the medium through which it passes. But he found that the maximum, in this case, did not exceed, or rather, that it did not quite reach, 3 feet; for, making use of a thick iron rod, 8 or 9 feet long, the lateral explosion, taken at the extremity of it, was about the same, as when it was taken at the end of a rod 4 inches from the jar; and not half so considerable as when taken at the extrémity of a rod one foot long. This, he

imagined, might be owing to the obstruction which the electric fluid meets with in passing even through metals; which appears, by his former experiments, to be much more considerable than was generally imagined.

On the whole, this remarkable experiment seems to be made to the most advantage in the following circumstances. Let the jar stand upon the table; let a thick brass rod, insulated, stand contiguous to the coating; and, near the extremity of this rod, place the body that is to receive the explosion. This body must be 6 or 7 feet in length, and, perhaps, some inches in thickness, or be connected with a body of those dimensions. Lastly, let the explosion be made with the discharging rod resting on the table, close to a chain, the extremity of which reaches within about an inch and a half of the coating of the jar. In this case, the operator will hardly fail of getting a lateral explosion of an inch in length; which shall enter and leave the insulated body, without making any sensible alteration in the electricity natural to it.

With large jars, containing 3 or 4 square feet of coated glass, bearing a very high charge, Dr. P. makes no doubt but that this experiment might be made to much more advantage; but, at the time that he was engaged in this investigation, he happened not to have any such jar, and therefore only used one that contained half a square foot of coated glass. If the interruption in the circuit, which is almost necessary in these experiments, be made by introducing a length of chain into it, rather than by making part of the explosion pass along the tube, there is a medium in the length of chain, that answers better than either a longer or a shorter circuit. In a long interrupted circuit, the electric matter seems to lose the impetus which it discovers in a short one. In all these cases, the electric charge seems to remain for a moment in the parts of the interrupted circuit; and therefore instantaneously rushes, in all directions as well towards bodies that are not placed along its passage to the jar, as those that are; but, when the same charge occupies a larger circuit, it has more room to expand itself, and is not so strongly impelled to desert it. He found, however, by repeated trials, that when he made use of 3 yards of brass chain in the circuit, there was a distance to which the lateral explosion would not reach. The same distance it also would not reach, when the circuit consisted of only one brass rod; but it reached it with great ease, when only half a yard of chain was used, even without any other interruption in the circuit. But it reached to a much greater distance, when the chain was very short, and the interruption was greater in other respects.

Dr. P. had imagined, that, since the body which had received the lateral explosion, contained, for a moment, more than its natural quantity; that if it were acutely pointed, some would escape, and that, on the return of the explosion, the body would be exhausted; but he found no such effect, though he

affixed fine needles to the bodies he made use of. The lightest pith balls placed near the extremities of these needles, were not in the least affected by the explosion. When he placed a number of brass balls, one behind another, the lateral explosion passed through them all, being visible in the intervals between each of them, and returned the same way, leaving them all in the same state in which it found them; and a great number of lateral explosions might be taken at the same time, in different parts of the circuit, some of them very near one another. It made no difference, whether the lateral explosion was received on a flat smooth surface, or the points of fine needles. In both cases the spark was equally long and vivid.

Dr. P. had no sooner completed these experiments on the lateral explosion, but he had a curiosity to see what kind of an appearance it would make in vacuo; since no other phenomenon in electricity resembles it. In all other cases, the electric matter rushes in one single direction; whereas, in this, it goes and returns in the same path, and, as far as can be distinguished, at the same instant of time; so that all the difference of the two electricities, which are so conspicuous in vacuo, must here be confounded. Accordingly he found, though the pump was not in good order, that he could perceive this explosion in vacuo, at the end of rods, placed several inches asunder; and when they were brought within about 2 inches, they seemed to be joined by a thin blue or purple light, quite uniform in its appearance. As these rods were made to approach, this light grew denser; but still exhibited no such variety, as is observed between the bodies that give and receive electricity, in the common experiments in vacuo. Dr. P. was pretty soon convinced, that uncoated jars could not be used to any more advantage in these experiments, than those that were coated; since the want of coating only operated as an interruption in the circuit, occasioning a difficulty in the admission of the charge on the outside of the jar. And, in all cases, the greater this difficulty of passage was made, provided the discharge was made at once, the more considerable was the lateral explosion, and the greater shock was given to the hand that held the discharging rod; which shock was nothing more than one of these lateral, explosions, issuing from the rod as part of the circuit. He concludes the account of these experiments with observing, that they may possibly beof some use in measuring the conducting power of different substances; since, the greater is since, the greater is the interruption in the electric circuit, occasioned by the badness of its conducting power, the more considerable, cæteris paribus, is the lateral explosion.

XIX. Experiments and Observations on Charcoal. By Joseph Priestley, LL.D., F. R. S. p. 211.

May be consulted in the author's collected works on different kinds of air.

XX. and XXI. Meteorological Observations for 1769, made at Bridgewater, Somersetshire; and at Mount's Bay, Cornwall, by Wm. Borlase, DD, F. R. S. Communicated by Dr. Jeremiah Milles, Dean of Exeter, and F. R. §. p. 228. These observations, are of the barometer, thermometer, winds, and weather, and quantity of rain, which last for the whole year, is 23.66 inches, at the former place, and 42.73 at the latter.

XXII. On the Manna Tree, and on the Tarantula. By Dominico Cirillo, M. D. Prof. Nat. Hist. at Naples. p. 233.

The manna tree, commonly called ornus by the botanists, is a kind of ash tree, and is to be found under the name of fraximus ornus, in Linnæus' Sp. Plant. This kind of fraxinus is very easily distinguished from the common fraxinus sive fraxinus excelsior, by the leaves, which are round at the top, subrotunda, integerrima. This tree very seldom grows to a considerable height, nor does it acquire a considerable bulk; in general it is from 10 to 20 feet high, the trunk is commonly of 5 or 6 inches in diameter, and the branches are pretty numerous, and irregularly spread: these dimensions however vary, if these trees are not crowded together, and have more liberty of growth. The manna tree is common, not only in Calabria and Sicily, but also on the famous mountain Garganus, situated near the old town of Sypontum on the Adriatic; and is mentioned even by Horace as an inhabitant of that mountain. In all the woods near Naples the manna tree is to be found very often; but, for want of cultivation, it never produces any manna, and is rather a shrub than a tree. The manner, in which the manna is obtained from the ornus, though very simple, has been yet much misunderstood by all those who travelled in the kingdom of Naples; and among other things they seem to agree, that the best and purest manna is obtained from the leaves of the tree; but this seems to be an opinion taken from the doctrine of the ancients, and received as an incontestible observation, without consulting nature. The manna is generally of two kinds; not on account of their intrinsic quality being different, but only because they are gotten in a different manner. In order to have the manna, those who have the management of the woods of the orni, in the month of July and August, when the weather is very dry and warm, make an oblong incision, and take off from the bark of the tree about 3 inches in length, and 2 in breadth; they leave the wound open, and by degrees the manna runs out, and is almost suddenly thickened to its proper consistence, and is found adhering to the bark of the tree. This manna, which is collected in baskets, and goes under the name of manna grassa, is put in a dry place, because moist and wet places will soon dissolve it again. This first kind is often in large irregular pieces of a brownish colour, and frequently is full of dust and other impurities. But when the people want to have a very fine manna, they apply to the incision of the bark, thin straw, or small bits of shrubs, so that the manna, in coming