The mechanism of the tempering of glass drops, applied to that of steel, is the most simple of all the hypotheses, and answers all its properties, which are these: 1. Tempered steel has a coarser grain. 2. It is increased in bulk. is harder and brittler. 4. By annealing it becomes less brittle.

3. It

Explanation: Steel made red-hot is filled and swelled, and its pores dilated, by the igneous matter. In this state, the cold water, into which it is thrown, compresses and closes the parts of the surface, while the imprisoned igneous matter dilates the pores within; thus the texture of steel becomes more compact by these two causes, while its pores are dilated. These large pores constitute the coarse grain of tempered steel. Its dilatation by the igneous matter, which could not be thoroughly condensed by the cold of the water, causes its augmented bulk; the close texture of the substance that surrounds the pores, and the imprisoned igneous matter, occasion its hardness and brittleness. Its recoction or annealing deprives it of this brittleness, and of a part of its hardness; because it opens this texture, which it relaxes at the expence of the neighbouring pores, and drives the igneous matter out of it.

Also the fermentation of acids and alkalis seem to be another corollary of the same principle.

First, It is pretty universally allowed, that the acid particles have the figure of small needles; and that alkalis are spheroidal or polyhedrous bodies, with a vast number of pores proper to admit the acid needles. Secondly, Experience shows, that salts are alkalised by fire, and that our juices are alkalised by heat, &c. What can the repeated action of the fire produce on salts, in order to alkalise them? it calcines them, blunts their points, and hollows them with a vast number of pores; and we see with the naked eye, that calcination has this effect on all bodies. In short, it converts an angular very solid body into a very porous and light spheroid; and this body is an alkali by the first supposition. Thirdly, calcination introduces, and generally leaves in the pores of the calcined body, after the operation, a great quantity of igneous matter. This matter is perceptible to the senses in the lapis bononiensis, which becomes a phosphorus by calcination; in lime-stone, which by calcination is furnished with so great a quantity of igneous matter, that in the effervescence, which is raised in it by throwing a little water on this stone, you may kindle sulphur or a match by it. The alkaline, or alkalised salts also, that is, those that are calcined, have their pores full of the igneous matter. Fourthly, such is the nature of the igneous matter, that it tears asunder whatever opposes its passage, and makes it fly off with a report. This principle is universally allowed: the effects of gunpowder, of volcanos and earthquakes, prove it; and to come nearer our subject, unnealed glass breaks in the air, and the lacryma Batavica does as much on breaking its small end.

Whereas an alkali is a spongious body filled with the igneous matter, and an

acid are points proportioned to these pores; these ought to be regarded as so many pegs or pins, which enter into the holes on the surface of the alkali, and fill them up exactly: by which the igneous matter is imprisoned; and by the preceding principle it bursts the alkaline globule with noise, and scatters around the acid pegs, in the same manner as it bursts the glass drop.

A mixture of an alkaline and acid liquor being composed of an infinite number of such particles that burst and broke to pieces, the liquor must take up more room, or swell. The particles of contained air being tossed about by all those little explosions, together with the neutral liquors, which are a vehicle to the salts, form the scum or froth; and the igneous matter, which gets out of the alkalis, and is agitated by the shocks of all these explosions, produces heat, drags with it the aqueous and other volatile particles, which form the steam.

Yet there are cold fermentations, because then, either the motion of the particles of fire, and their burstings are inconsiderable; or because these particles fly off easily by a direct motion. Further, at this day that we have it in our power to be convinced, that the brush or stream of electric matter is very cold, nobody will be surprised that a stream of the matter of fire may produce cold.

If all the alkalious corpuscles bursted at once, the fermentation would last but an instant: but as the acid liquor requires a certain space of time, to penetrate the whole alkaline liquor, and fill the pores of the alkalious corpuscles, the fermentation is performed successively in a certain number of corpuscles at a time, till they are all broken: and this succession constitutes the duration of the fermentation; which ceases when there are none of the alkalis left entire.

These principles not only serve to explain the fermentation which results from the mixture of acids and alkalis, but also almost all the motions of this kind, which are occasioned by the mixture or penetration of 2 or more substances. For example; lime, which we have mentioned above as a body filled with the matter of fire, and which produces an effervescence capable of lighting sulphur, if water be thrown on it; lime then produces this effect, only because the particles of water, which enter into its pores, have a tendency to shut up the igneous particles more closely. It is by a mechanism entirely similar, that Homberg's phosphorus kindles into flame, on being exposed to the air: it is on this principle also that a mixture of spirit of wine and water acquires a new degree of heat; and so of other phenomena of this nature.

On the Electricity of Glass, that has been Exposed to Strong Fires. By Mr. Prof. Geo. Matthias Bose of Wittemberg. N° 492, p. 189.

It seems that a glass ball, which has often been employed for violent distillations, and other chemical operations, sends forth the electricity incomparably more strong than any other glass, which never since its making had been ex

posed to a violent fire. As I am the first, says Mr. B. that has mentioned this notable circumstance, be pleased to let me have the honour of this improvement in the Phil. Trans.

Of an Extraordinary Rainbow, observed July 18, 1748. By Peter Daval, Esq. Sec. R. S. N° 493, p. 193.

On Monday July 18, 17-18, about a quarter before 7 in the evening, the weather being temperate, and the wind about N.N.w., walking in the fields beyond Islington, Mr. D. saw a distant rainbow, which appeared to take in a large portion of the heavens; but had nothing remarkable, and vanished by degrees. Continuing his walk, about 20 minutes after the disappearing of the first rainbow, a rainy cloud crossed him, moving gently with the wind, which exhibited a more perfect and distinct rainbow, than he had ever before scen; in which he could plainly distinguish all the secondary orders of colours noticed by the late Dr. Langwith, in his letters published in the Philos. Trans. N° 375, that is, within the purple of the common rainbow, there were arches of the following colours: 1. Yellowish green, darker green, purple. 2. Green, purple. 3. Green, purple.

Of the Present Condition of the Roman Camp at Castor in Norfolk, with a Plan of it; also a Representation of a Halo or Mock-Sun observed July 11, 1749. By Mr. Wm. Arderon, F.R.S. N° 493, p. 196.

The town of Castor is at present in a very low condition, containing no more than between 20 and 30 small cottages. It stands about 4 miles south-west of Norwich, and by tradition, and some learned authors, is supposed to have been a considerable city, out of whose ruins Norwich took its rise. However, at this day, excepting the camp, not the least trace or footstep of any thing remarkable is left remaining.

The camp itself lies near a furlong south-west from the town of Castor, and leads you by a gentle descent down to the little river Wentsum, which swiftly glides close to the end of it, and doubtless at the first forming of the camp was designed to be part of the fortification on that side, as well as to supply the army with water, and to bring up such things as they wanted from the sea, if their communication by land should at any time be impeded. This river is by some called Taus, or Tese: but probably it did not formerly take that name till it approached the Roman camp at Teseburgh, 3 or 4 miles higher.

We are told by tradition, as well as by some learned authors, that the sea came up to this camp; and indeed every intelligent observer must confess, that the marine bodies found in every part of Norfolk, on the highest hills, as well as in the lowest pits and valleys, are indubitable proofs, that at some time or

other the sea must have covered this whole country: but then we may be assured by the present condition of this camp, that the sea has not exceeded the level of it since it has been in being, which if we credit several of our ancient historians, it was upwards of 1700 years ago. It may therefore serve to prove, that the sea since that time has not exceeded these bounds, and that the fossils dug up above this level are more ancient than it, though we have no proper data to discover how long before the sea had passed this height.

The figure of the camp is not a square, but a parallelogram, whose 2 longest sides are each 440 yards, and its ends or 2 shorter sides 360 yards each. These are its dimensions withoutside the rampart and ditch; but within the length is 392 yards, and the breadth 264. The breadth of the fosse and rampart, in some places where it remains most perfect, was 48 yards, though in others not above 30. And the whole ground taken up, including the ditch and rampart, is 32 acres, 2 roods, and 36 poles; or the area within the ditch and rampart 21 acres, 1 rood, 21 poles.

Three sides only of this camp have been fortified with a rampart, whose upper part was faced with a thick and strong wall, made of lime and flints; of which wall there are still remains in several places of the rampart, besides a very deep ditch, that seems to have been most considerable on the east and south sides. The wall on the north side appears to have been built at 2 different times; that ́ is, it seems to have been raised higher than it was built at first, at some distance of time afterwards; for a parting may be observed at a certain height running from end to end.

The ruins of 2 old towers still remain, one of which stood on the north side, and the other at the west end; the last of which is at present the most considerable of the two. They were both built in a manner perhaps peculiar to the Romans at that time, and which it may not be improper to describe. They began first with a layer of bricks laid flat as in pavements; on that they placed a layer of clay and marl mixed together, and of the same thickness as the bricks; then a layer of bricks, afterwards of clay and marl, then of bricks again, making in the whole 3 layers of bricks and 2 of clay: over this were placed bricks and lime 29 inches, the outside being faced with bricks cut in squares, like the modern way of building in some parts of Norfolk, then bricks and clay, again stratum super stratum, as high as the old ruins now remain standing.

The mortar is found extremely hard at this day: it is a composition of lime, sand, and ashes, and so compact, that he could by no means break a piece of it, of an inch diameter, from the base of one of the towers at the east gate, but on striking it with a sharp flint it flew off in dust.

The Roman bricks which he examined, were made of 2 different sorts of clay

mixed; when burnt, one appears red and the other white: at the time of viewing them, they were exceedingly hard and solid, and far superior to any thing of the kind now made with us. Perhaps they are little worse than when they were first laid down. These bricks were made without the assistance or addition of sand, as is too much the practice at present here in Norfolk: for when sand enters the composition in any considerable proportion, it renders the bricks friable, soft, and rotten, subject to be broken or ground to pieces with the least motion or pressure. The length of these bricks is 17 inches, or a Roman foot and half; and their breadth 11 inches, or precisely a Roman foot: which may serve as some proof that the Roman measures, handed down to us by several authors, are right, and may likewise inform us of the proportionable stature of man at that time. The thickness of these bricks is 1 inch.

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The great number of Roman medals that have been, and still are found in and about this camp, are a matter of great wonder. One lady who lives near the place, has it seems picked up at least 100; and several are daily gathered up by boys, and sold to strangers who come to visit the place. That these pieces have been used as money seems exceedingly clear, from their different degrees of fection; some being worn almost quite smooth, others having imperfect busts without letters, and others again having both the busts and inscriptions fair and legible, which could only happen from their different wear as money.

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A particular kind of halo was observed at Norwich, on the 11th of July 1749, at 5 o'clock in the evening: the colours were exceedingly vivid, and the centre of it, contrary to what he ever yet saw, was not in the sun, but in the zenith. The sun's rays shone through the clouds at the same time, as they frequently do when the sun is near the horizon.

Part of a Letter from Leonard Euler, Prof. Math. at Berlin, and F. R.S. To the Rev. Mr. Caspar Wetstein, concerning the Gradual Approach of the Earth to the Sun. Dated Berlin, June 28, 1749. Translated from the French, by S. T., M.D., F.R.S. N° 493, p. 203.

M. le Monnier writes to me, that there is at Leyden an Arabic manuscript of Ibn jounis (if I am not mistaken in the name, for it is not distinctly written in the letter), which contains a history of astronomical observations. M. le Monnier says, that he insisted strongly on publishing a good translation of that book. And as such a work would contribute much to the improvement of astronomy, I should be glad to see it published. I am very impatient to see such a work which contains observations, that are not so old as those recorded by Ptolemy. For having carefully examined the modern observations of the sun with those of some centuries past, though I have not gone farther back than the 15th century, in which I have found Walther's observations made at Nuremberg; yet I have ob