1

duce a very great one at F, and therefore the index will turn very sensibly on the plate.

The proportions of the rod and lever are discretionary; his rods, both of steel and brass, were 3 feet long in one solid piece, but they had each a point or cone of steel 1 inch high, that screws on the top at A. The lever has 4 inches from E to A, 1 inch from A to G, and 12 inches from G to F; the distance of G above c is 1 inch, the brass pulley н is inch diameter; all the other parts of the machine are of oak. The main support or pillar pa is 1 inch square, 24 feet high, and at bottom is let through a groove at a made in a large heavy block or pedestal of wood rs. In this groove the pillar may be raised higher or lower to adjust the height of the pillar to the situation, which the bottom of the rod AB may require in different experiments; and it is to be fixed in that place by a screw at T, which goes through the front of the block, and presses against the bottom of the pillar.

Fig. 3 represents the dial plate, or front of the plate marked мNo in fig. 2. It is a plate of brass, with strong paper glued on it, and may be of any size, Dr. M.'s is 11 inches over. AB is the hand or index, which slips on very stiff on the axis c, that carries the pulley н in fig. 2. The outer circle is to be left wide enough to contain the chemical characters or marks to be made on it; the arch DE contains the divisions of Fahrenheit's mercurial thermometer; and the arc FG those of Reaumur, or the spirit of wine thermometer.

To adjust this instrument for use, place the bottom of the rod в in fig. 2, immersed up to the mark in cold river or rain water, in a vessel proper to be set over the fire; and when it has boiled for a quarter of an hour, turn the index AB in fig. 3, till it stands in the horizontal position, as at в, being the point of boiling-hot water, and which answers to division 212 on Fahrenheit's arch. Then take it out of the water, and dry it, by holding it a little over the fire: and now great care must be taken, that nothing alters the situation of the index on the axis, a nut to screw on the axis at c may be a good way to keep it fixed. If the instrument be left to cool in the air, the index will fall below B, showing the degrees of cold, or less heat than boiling water; and if put into melting tin, lead, &c. it will show the degrees of heat above boiling water. A brass rod will serve for an instrument to measure the greatest degrees of cold, and all the degrees of heat, to the melting of silver or gold, but to make one to measure greater degrees of heat, the rod must be of steel, or the finest iron. A rod of brass, according to Dr. Muschenbroek's experiments, was found to lengthen 377, when one of iron lengthened only 230 parts. An iron rod, being regulated by boiling water as above directed, will measure not only the heat of melted tin and lead, but of silver, gold, and copper, and will even show the degree when iron itself begins to melt, which will be the greatest degree of elongation of the rod

just before its bottom runs; and he imagines, that an instrument may be con→ structed with supporters, and a rod made of tobacco-pipe clay,* which, being regulated by boiling mercury, for it must never touch water, may be adapted to measure still greater degrees of heat, till the materials themselves melt into glass.

Dr. M. advises, that not only the scale of this kind of thermometer, but also of all others, be determined by experiments, without regarding any equality as to measure between the divisions, and that in every individual that shall be made; for a difference in the length and thickness of the rods will make a difference in the scale, as much or more than the inequality in the cavity of the stem, or glass tube of other thermometers, which can never be just, if applied to a scale whose divisions are made equal; unless the cavity of the stem be perfectly equal, which it is impossible for any workman to do, and which is very seldom, if ever, hit on by chance. Therefore, in these instruments, let the point в in fig. 3, or the horizontal position of the index, be the situation of the index when the rod has stood a quarter of an hour in boiling water; there mark boiling on the outer circle; on Fahrenheit's arch mark 212. Then set the machine up to the mark + into melting tin, which is the metal that melts easiest. When the rod is arrived to its greatest elongation in that metal, inscribe the character on the outer circle; do the like with lead, and set the character ↳ at it. At the boiling of mercury put the mark, and on Fahrenheit's arch mark 600, the utmost extent his mercurial instruments can measure: then proceed to the melting of silver, and set the mark; at the melting of gold place the mark ; at the melting of copper place the mark ; at the melting of iron place the mark ♂, the most difficult to melt of all metals.

As the divisions pointed out by the index will be different with rods of dif ferent metals or substances, we may make different circles on the plate for the range of the different rods, and mark them; the iron rod, the brass rod, the clay rod; and set the several marks above specified on each circle apart; or, to avoid confusion, we may have a different instrument for each kind of rod.

According to Fahrenheit's scale, the heat of the strongest sunshine is at about 80; spirit of wine boils at 176; water at 212; the lixivium of salt of tartar at 240; spirit of nitre at 242; oil of vitriol at 546; quicksilver at 600.†

As all chemical digestions, where an equable heat is to be continued for some time together, will come in between hot sunshine and the boiling of quicksilver,

This idea, of using argillaceous bodies as thermometers, for measuring very high degrees of heat, was afterwards further acted on by Mr. Josiah Wedgwood, in vol. 72, &c. of the Philosophical Transactions.

+ See Augustin. Grischow Thermometria comparata accuratius, et Harmonica. Berolini 1740, 4to. p. 10.Orig.

a thermoscope of that range will be sufficient for common uses; and therefore one fitted with a brass rod will answer these purposes.

In large furnaces for running down ores, or melting great quantities of metal together, it is not possible to place such an instrument; but then in lead and tin there may be small outlets contrived, into which some of the melted metal may be permitted to flow, and remain in contact with the same body of metal. within, where the instrument may be placed; and for placing a thermoscope in iron, copper, or glass furnaces, there may be a place contrived, which shall not open into the furnace, but have the thickness of a stone or brick left between, on which the instrument may be placed; and though in such a situation it will not measure the actual heat within the furnace, it will always give the relative or comparative heat in the like circumstances at different times, and so show how to regulate the heat within.

Though a chemist shall have one of these instruments to measure the heat he may have used in any experiment, and have noted down the several degrees made use of, and the time each lasted, he still labours under another difficulty, which is the not being able to command any required heat, and that it shall last a certain required time, unless it be below that of boiling water, which may be procured and continued by various contrivances of lamps, either of spirits, or of oil; but how to continue a fire for 12 or 24 hours together, without attendance, which shall continually keep quicksilver boiling, lead in fusion, or may be let down so low as not to exceed the heat of sunshine, and then be raised again, and that without letting out the fire, or moving the vessels, may seem almost impracticable; but by an improvement of the furnace the ancient chemists called their athanor, he thinks it may be effected.

Remarks on the foregoing paper. By the Rev. Stephen Hales, D. D. p. 693.. -What I intended to do, says Dr. Hales, was only this, viz. to get a leaden wire, of such a size and strength as to bear its own weight, to have it as long as the longest gun-barrel I could procure, and to have it sustain a lever as above; then to pour boiling water into the barrel, for a long time, till the lever rises no more; the water to have vent at the bottom, yet so as to have the gun-barrel always full of water; the breech-pin to be out, and the leaden rod to rest on a piece of wood set upright, according to the course of its fibres, not sidewise. To give at the same time to a mercurial thermometer the heat of boiling water. Then to take the freezing point of the leaden and mercurial therinometers; and afterwards to graduate all the intermediate degrees, from the mercurial thermometer on the leaden thermometer, as they occur. Thus a standard thermometer may be made to graduate others by; but I need not now set about it, since that is done above.

"All solid bodies are observed to contract with cold." I have found that

wood does not contract or dilate lengthwise with heat or cold. I am told that Mr. George Graham is about making this experiment, as I am also, in order to regulate pendulums.-I fear that Boerhaave's wet linen, which is so thin, may begin to freeze before all the mercury or spirit of wine in the ball of the thermometer has the same degree of cold; though hanging there long before and after freezing will bring it pretty near." A rod of iron 3 feet long will have about inch increase," or TM part.

Continuation of a Paper concerning Electricity. By William Watson, F. R. S. printed in these Trans. N° 477,* p. 695.

As water is a non-electric, and of consequence a conductor of electricity, there was reason to believe that ice was endowed with the same properties. On making the experiment, Mr. W. found his conjectures not without foundation; for on electrifying a piece of ice, wherever the ice was touched by a non-electric, it flashed and snapped. A piece of ice also held in the hand of an electrified man, as in the beforementioned processes, fired warm spirit, chemical vegetable oils, camphor, and gunpowder prepared as before.

Prop. 1.—In common with magnetism, electricity counteracts, and, in light substances, overcomes the force of gravity. Like that extraordinary power likewise, it exerts its force in vacuo as powerfully as in open air; and this force is extended to a considerable distance through various substances of different textures and densities.

Corol:-Gravity is the general endeavour and tendency of bodies towards the centre of the earth; this is overcome by the magnet with regard to iron, and by electricity with regard to light substances both in its attraction and repulsion; but Mr. W. has never been able to discern that vortical motion, by which this effect was said to be brought about by the late Dr. Desaguliers and others, having no other conception of the manner of its acting than as rays from a centre, which indeed is confirmed by several experiments: one of which, very easy to be tried is, that if a single downy seed of cotton-grass be dropped from a man's hand, and in its fall come within the attraction of the rubbed tube, the down of this seed, which before seemed to stick together, separates, and forms rays round the centre of the seed. Or if you fasten many of these seeds with mucilage of gum Arabic round a bit of stick, the down of them, when electrified, which otherwise hangs from the stick, is raised up, and forms a circular appearance round the stick. As these light bodies are directed in their motions only by the force impressed upon them, and as their appearance is constantly radiatim, such appearance by no means squares with our idea of a vortex.

*See p. 151 of this volume.

Prop. 2.-In common with light, electricity pervades glass, but suffers no refraction from it; having, from the most exact observations, found its direction to be in right lines, and that through glasses of different forms, included one within the other, and large spaces left between each glass.

Corol. This rectilineal direction is observable only as far as the electricity can penetrate through unexcited originally-electrics, and those perfectly dry: nor is it at all material, whether these substances are transparent, as glass, semidiaphanous, as porcelain, or thin cakes of wax; or quite opaque, as thick woollen cloth, as well as woven silk of various colours; it is only necessary that they be originally-electrics. But the case is widely different with regard to non-electrics; in which the direction, given to the electricity by the excited originally-electric, is altered as soon as it touches the surface of a non-electric, and is propagated with a degree of swiftness scarcely to be measured, in all possible directions, to impregnate the whole non-electric mass in contact with it, or nearly so, however different in itself, and which must of necessity be terminated by an originallyelectric, before the electricity exerts the least attraction; and then this power is observed first at that part of the non-electric the most remote from the originallyelectric. Thus, for example, by an excited tube held over it, leaf-gold will be eattracted through glass, cloth, &c. held horizontally in the hand of a man standing on the floor, and this attraction is exerted to a considerable distance. On the contrary, the rubbed tube will not attract leaf-gold, or other light bodies, however near, through silver, tin, the thinnest board, paper, or any other nonelectric, held in the manner beforementioned. But if you rub the paper over with wax melted, and by that means introduce the originally-electric therein, you observe the electricity acts in right lines, and attracts powerfully.

Prop. 3.-Electricity, in common with light likewise, when its forces are collected, and a proper direction given to it on a proper object, produces fire and flame.

Corol. The fire of electricity is extremely delicate, and sets on fire, as far as yet experienced, only inflammable vapours. Nor is this flame at all heightened by being superinduced on an iron rod, red-hot with coarser culinary fire, as in a preceding experiment; nor diminished by being directed on cold water. However, to know if this flame would be affected by a still greater degree of cold; he made an artificial cold, by which the mercury, in a very nice thermometer adjusted to Fahrenheit's scale, was depressed in about 4 minutes, from 15 degrees above the freezing point to 30 degrees below it: that is, the mercury fell 45 degrees. From this cold mixture, when electrified, the flashes were as powerful, and the stroke as smart, as from the red-hot iron. This experiment seems to indicate, that the fire of electricity is affected neither by the presence nor absence of other fire. For, as red-hot iron, by Sir Isaac Newton's scale of heat, is fixed 3 G

NOL. IX.