Allowing therefore the clock to get 1m 595 in a mean solar day, on mean solar time, in 23h 56m 4s of mean solar time, or in a sidereal day, it will get only 1m 58 on mean solar time: but a clock, adjusted to mean solar time loses 3m 56s by the stars in a sidereal day, therefore the clock loses 1m 57s of sidereal time, in one revolution of the stars, which agrees exactly with what he found by the transits, v, α, and Leonis.

The same clock at Greenwich, with the pendulum adjusted to the same length, lost 11s on the stars in a sidereal day. Therefore the force of gravity at Greenwich is to the force of gravity at St. Helena as the square of 23h 59m 49s to the square of 23h 58m 253: 10000000: 9975405. The extent of the vibrations of the pendulum here, as well as in England, is exactly 1° 45′ on each side of the perpendicular, according to the divided arch, which is at the bottom of the pendulum.

Mr. M. excuses himself from attempting to deduce any consequences at present from the above observations, either with respect to the law which the force of gravity observes in its changes in different latitudes, or with respect to the figure of the earth, which it has been supposed might be determined from experiments of this kind alone, independently of any others, the great Sir Isaac Newton having himself set us the example. If the body of the earth were homogeneous throughout, not only the figure of the earth, but also the law of the variations of gravity in different latitudes, would be given, and would be the same as Sir Isaac Newton has described them. But if the earth be not homogeneous, and there seems great reason, from late experiments, to doubt if it be so, we can form no certain conclusions concerning the figure of the earth, from knowing the force of gravity in different latitudes; as this force must depend not only on the external figure, but also on the internal constitution and density of the earth. Many more experiments, not only of the kind, but also of other different kinds, may be necessary before we shall be able to infer any thing with certainty concerning the internal constitution of the earth, or even to determine its external figure. But every experiment is useful which tends to throw a light over this intricate subject, and to show the perfect agreement of the laws of nature with the actual constitution of things.

LXVII. Observations on some Gems Similar to the Tourmalin. By Mr. Benjamin Wilson. F.R. S. p. 443.

Mr. W. here mentions that he had met with several gems of different sizes and colours, that resemble the tourmalin in regard to electrical experiments. The most beautiful of them were something like the ruby, others were more pale, and there was one inclining to the orange colour. In point of hardness and lustre they were nearly the same with the topaz.

Six of these gems were cut brilliant fashion, and 3 retained their natural

shape. Of the 6, when heated properly, and while they were cooling, 3 were electrified plus, and 2 minus at the table surface, and at the collet or opposite - surface the 3 were electrified minus and the 2 plus. The 6th, which was the largest, and of an oblong shape, appeared to be electrified plus near one end, and at the other end, in the opposite part, minus. These instances he thinks are further proofs of the law observed in the tourmalin, viz. that the electric fluid flows to or fro, in one invariable line, according to the circumstances attending the experiment. See the letter to Dr. Heberden, Phil. Trans., vol. 51.

From the contrary appearances happening with gems of the same shape, it is now abundantly evident that the direction of the fluid does not depend on the external figure of the gem, but on some particular internal make or constitution of it. And that there is some such natural disposition in all gems affording these appearances, may be collected from another curious specimen of the tourmalin kind; which is green, and formed in long slender crystals with several sides, many of which are found sticking together, and are brought from South America,

Mr. E. M. da Costa, member of the R. S., furnished Mr. W. with a parcel of these uncommon crystals the 12th of November 1761, and desired he would try whether they afforded the electric effects of the tourmalin. They were examined the same evening, when he was agreeably surprized to find them, not only like tourmalins in regard to electric appearances, but that the direction of the electric fluid moving in them, is always along the grain or shootings of the crystals; one end being electrified plus, and the other end minus. And that the fluid is more disposed to pass in that direction than in any other, may be further collected from what has been observed on the grain of the loadstone by Dr. Knight; for though the magnetic poles of a natural loadstone may be varied. in any direction, yet the same loadstone admits of being made much more magnetical along the grain than across it.

LXVIII. Observations on the Tides in the Straits of Gibraltar. By Henry More, Esq. p. 447.

That a very strong tide, at the rate of some knots, sets from the strait into and out of Gibraltar bay, from Cabrita and Europa points, is notorious. That this stream out and in by Europa is a mile or better wide, he had frequently observed. At the same time that the tide has been pouring into the bay, round Europa, he had remarked, both from the high shore and in boats, another stream in the offing, going the contrary way.

As a further confirmation of his idea, relating to the mid-stream: being on guard at Europa, in the forenoon, there came a Spanish xebeck from the west, with little wind, and in time was becalmed right off a rock he sat on; where he

continued almost the whole day to observe her, driving back again with a midstream, to appearance half-channel over, and edging outwards towards the Barbary side; when about 7 in the evening, with little or no wind as before, she returned at a great rate, and so continued till night and distance hid her in the Mediterranean.

It seems to be Mr. M.'s opinion that there are different streams or currents setting opposite ways in the straits at the same time.

LXIX. The Case of a Young Man Stupified by the Smoke of Sea-coal. By Dr. Frewen of Sussex. p. 454.

Wm. Colebrook, 17 years of age, was left alone to take care of his master's vessel in Rye harbour, the 4th of June 1761; and shutting up all close, at 9 o'clock in the evening, he laid himself down to sleep in a small cabin, where there had been a sea coal fire, which was not properly extinguished, and the chimney place being stopped, it soon became full of smoke; the effect of which, when the people came on board next morning, proved to have been so powerful as to render him totally deprived of all the sensible motions of the body, excepting those of the heart and lungs. The cause of this stupor being presently suspected, he was brought out upon the deck, in hopes the fresh air would prove of service; but neither that, nor bleeding, blistering, or any other applications they made use of, assisted him in the least under this torpid situation. Being brought home to his master's house about noon, Dr. F. visited him, and found him in the same soporous, apoplectic state, with a feeble pulse, respiration laboured and difficult, a rattling in his throat, and utterly void of all sensation. He appeared much like one he had seen who had taken an over-dose of opium, and died of it.

Dr. F. strongly recommended plunging this patient into a cold bath, which being complied with, and done as expeditiously as it could be, was attended with a success even beyond his expectations. Immediately on the immersion he opened his eyes and mouth, and shut them again. He was then instantly put to bed naked between the blankets; and in a very few minutes time a very great and universal sweat came on him, which continued for many hours. In the evening he was first perceived to move a little, seemingly as if disturbed by the roughness of the blankets stimulating his skin: a while after he opened his eyes, and looked a little about him in a confused manner. Some time after, he became more sensible and spoke; but could only give the short answers, yes and no. His respiration was still difficult and very laborious; but his pulse was stronger and fuller; on which account Dr. F. ordered a little blood to be taken away; and he took frequently a very little at a time, of some sweet oil. For his ordinary drink, Dr. F. directed boiling water poured upon bread, with a little white wine, lemon juice, and sugar,

of which at first he took but very little at a time, and afterwards more in quantity as he could get it down.

The next day Dr. F. found him much better, when he sat up, talked, and drank some tea, His breathing was easier, but he complained of a short troublesome cough and hoarseness; for which he ordered him a smooth pectoral linctus and a lenient purging draught was also given him, which had the desired effect. He continued getting better for a day or two, when Dr. F. called on him again, and finding his cough and hoarseness still remain, with a little shortness of breath, he directed him pills of millepedes and gum ammoniac made up with bal. sulph. to be taken twice a day, drinking warm milk after them; by which means he got perfectly well and went to sea in 12 days.

LXX. A Letter from Benjamin Franklin, LL. D., and F.R.S. Dated CravenStreet, Feb. 4, 1762. p. 456.

Mr. Canton did me the favour to show me the ingenious experiments he has described in the inclosed letter. They succeeded perfectly as he has related them; and I imagine the communication of them must be agreeable to the curious in this branch of natural knowledge.

LXXI. A Letter from John Canton, M.A., and F.R.S., to Benjamin Franklin, LL.D. and F.R.S. containing some Remarks on Mr. Delaval's Electrical Experiments. p. 457.

Mr. Delaval, in his curious electrical experiments, found that Portland stone, common tobacco-pipe, &c. would readily conduct the electrical fluid, when very hot, or when quite cold: but were non-conductors in an intermediate state. As no one has yet attempted to account for this, Mr. D. submits the following solution to Mr.F.'s judgment

The stone tobacco-pipe, wood, &c. I apprehend, says Mr. D. conduct when cold, by the moisture they contain in that state; when their moisture is evaporated by heat, they become non-conductors; and when they are made very hot, the hot air at, or near their surfaces, will conduct and the bodies appear to be conductors again.

To prove that hot air will conduct the electrical fluid, let the end of a poker, when red-hot, be brought but for a moment within 3 or 4 inches of a small electrified body, and its electrical power will be almost if not entirely destroyed. And if excited amber, &c. be held within an inch of the flame of a candle, it will lose its electricity before it has acquired a sensible degree of heat.*

I have observed also, that the tourmalin, Brazil topaz, and Brazil emerald, will give much stronger signs of electricity while cooling, after they have been held about a minute within 2 inches of an almost surrounding fire, where the air is a conductor, than they ever will after heating them in boiling water. And if both sides of either of those stones be equally heated, but in a less degree than

VOL. XI.

ΑΙ

That glass is a conductor in damp weather, on account of the moisture on its surface, is well known; as also that warming it a little will render it a non-conductor; and that a great degree of heat will make it seem to be a conductor again. Now tobacco-pipe, wood, &c. will not only attract the moisture of the air to their surfaces, but will also absorb it; whence they are conductors in dry weather; and require more heat than glass, as well as a longer continuance in it, to render them non-conductors. It is remarkable that tobacco-pipe after it begins to cool, will become a conductor again sooner than most other substances and much sooner than wood. The cause of this appears to me to be the tobaccopipe's absorbing the moisture of the air faster than most other substances, and much faster than wood; for the surfaces of tobacco-pipe and wood being wetted, the surface of the wood will continue wet much longer than the surface of the tobacco-pipe.

That tobacco-pipe does not become a non-conductor by a particular degree of heat without evaporating its moisture is evident, from the following experiments. If 3 or 4 inches of one end of a tobacco-pipe, of more than a foot in length, be made red-hot, without sensibly heating the other end, this pipe will prove a ready conductor, through the hot air surrounding one part of it, and the moisture contained in the other; though some part of it must have the degree of heat of a non-conductor. But if the whole pipe be made red-hot, and suffered to cool till it has only superficial moisture enough to make it a good conductor, and then 3 or 4 inches of one end be again made red-hot, it will become a non-conductor. And if a nail be placed at or near each end of a longish solid piece of any of the absorbent bodies ahove-mentioned, so that the point of each nail may be about half the thickness of the body within its surface; this body by heat may be made a non-conductor externally or superficially, while it remains a good conductor internally : for the electric fluid will pass readily from one nail to the other, through the middle of the body, when it will not pass on its surface; and even when the internal parts of the body are in an equal degree of heat with the external; as they must soon be after it begins to cool. But if the same body be exposed for a short time, to a greater degree of heat than before; or if it be kept longer in the same heat, it will become a non-conductor entirely.

P. S. Having formerly observed that the friction between mercury and glass in vacuo, would not only produce the light of electricity, as in the luminous barowill make the surrounding air a conductor, the electricity of each side, whether plus or minus, will continue so, all the time the stone is both heating and cooling, but will increase while it is heating, and decrease while it is cooling. Whereas, if the heat be sufficient to make the surrounding air conduct the electric fluid from the positive side of the stone to the negative side of it, while heating; the electricity of each side will increase, while the stone is cooling, and be contrary to what it was, while the stone was heating. See the Phil. Trans. vol. LI. p. 403 and 404.—Orig.