fhall add that the two pieces which we have taken the very allowable liberty, in our affumed character of conjurors, to call a guinea and a fhilling, are not ftrictly what they appear to be. Previous to the performance, two guineas and two fhillings are ground down on one fide to about half their usual thickness. Each of the guineas, thus diminished, is neatly riveted to one of the diminished fhillings; fo that each of the two pieces, when laid on the palm of the hand, appears to be either a guinea or a fhilling, according as the one or the other moiety is uppermoft. The trick is eafily performed by turning them over, in the act of fhutting or opening the hand.

This is the only fpecimen that we shall give of the legerdemain fo abundant in this performance. Laying afide therefore our cap and wand, we shall proceed to give an account of the diftribution of the other matter contained in this work; adding occafionally a fpecimen or two of the Author's recreations.

The first volume contains a variety of problems, experiments, or recreations, depending on numbers, of which tricks with cards, as ufual, conftitute no inconfiderable part. To these, as connected with the doctrine of combinations, are added different methods of writing in cypher, and decyphering.

In the fecond part of this volume the Author exhibits various experiments relating to Mechanics. The fubject of the 51ft recreation is A clock to go perpetually, by the influence of the celeftial bodies."

A common clock is to be placed near a wall against which the tide flows. To each of the barrels, round which the ftring that carries the weight is wound, there muft hang a bucket, and into that, when the tide rifes to a certain height, the water runs, by means of a pipe fixed in the wall. The bucket then overbalancing the weight, defcends, and winds up, the clock; but when it comes to a certain depth, it is taken by a catch fixed in the wall, which, by turning it over, difcharges the water. The weights of the clock then defcend in the ufual manner, and the buckets are drawn up.

Now as this clock is kept in motion by the tide, and as the tide proceeds from the influence of the fun and moon, it neceffarily follows, that the motion of the clock proceeds from the fame cause; and that as long as the parts of the machine remain, motion will be perpetual-though not, as the Author afterwards obferves, in the fenfe ufually affixed to the term by the mistaken advocates for a perpetual motion; but according to the vulgar acceptation of the phrafe, in which sense every mill driven by a conftant ftream, or every fmoke-jack moved by a constant fire, may be faid to be a perpetual motion.

In the fecond volume are contained fome of the many amufing experiments with which the fcience of Optics furnishes the experi

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experimental philofopher. Of these we fall particularife a cu rious deception, which we fhall endeavour to explain without. the ufe of figures. For the defcription of it the Author quotes M. Guyot, but it was originally published by Father Bonaventure, author of the Amusemens Philofophiques. We fhall col-lect the principal circumftances of the experiment, from the account given of it by the original author,

If a vial half full of water be held upright before a concave mirror, and beyond its focus, the image of the vial will be feen inverted, before the mirror; but the water contained in the body of the vial, instead of being inverted likewife, or appearing uppermost in the image, will there appear to occupy the lowest part, or the space between the mouth of the vial and its middle. If the bottle be held inverted, with its mouth flopped, its image will appear erect, but the water will feem to occupy the space between the bottom and middle of it. In both cafes, that part of the vial which contains the water will appear empty, in the image, and vice verfa.

If, while the vial continues inverted, the water be fuffered to drop flowly from its mouth, the image of the vial will seem to fill in proportion as the vial itfelf is emptying: but at the inftant when the latter becomes perfectly empty, the illufion fuddenly ceafes, and the image appears perfectly empty likewife. Further, if a few drops of water fall from the protuberance in the bottom of the inverted vial, they will, in the image, exhibit the appearance of fo many bubbles of air, rifing from the bottom of the vial to the furface of the water contained in it.

, Father Bonaventure's conjecture concerning the cause of thefe fingular deceptions is ingenious, and appears to be well founded. We have always, he obferves, been fo accuftomed to fee water occupy the loweft place in a vefil; and at the fame time the difference in appearance between the full and the empty part of the vial is fo little, on account of the purity and perfect tranfparence of the water, that the mind habitually fees the fluid where it is not, and does not fee it where it really is, notwithstanding all our reafonings and reflections to the contrary.

This deception, as we have obferved, may generally be conquered, and the water be feen in its true place, or in contact with the bottom of the veffel, when its image is inverted, by trying the experiment with a wide mouthed glass tumbler, efpecially on fhaking the veffel. During the agitation of the water, its apparent lower furface in the image, fuppofing that

* See the Appendix to our 29th vol. page 490, and to vol. xliii. page 533.

the

the illufion fhould, ftill continue, will be feen to remain unmoved, and never to defcend below, the mouth of the gial, as it will feem to the fpectator that it ought to do, were it really the image of the water.

-As few perfons are in poffeffion of concave mirrors, we fhall, add that thefe experiments will equally fucceed, if the vial be viewed through two convex lenfes (two of the eye-glaffes of a common telescope, for inftance, or two fpectacle glaffes of a fort focus) placed at the diftance of their common focus, fo as to exhibit objects in an inverted fituation.

Under this divifion of the work, and in the 42d recreation, the Author gives a fhort account of Colorific Mufic. Thofe who are not converfant in foreign philofophical literature may perhaps be startled at the fingularity of the term. As this fpecies of melody is very little known in this country, we fhall give a defcription of the grounds of it; not on account of the thing itself, but because we confider it as a fingular phenomenon in the intellectual world, that a philofopher, and a man of ingenuity, Father Caftel, who was the inventor of this propofed gratification of the eye, could poffibly bestow so much thought, and fpend fo much time as he is known to have done, in the purfuit of fo unattainable a phantom.

Sir Ifaac Newton firft obferved that the length of the spaces occupied by the feven primary colours in the folar Spectrum happened to correfpond, not to the lengths, as our Author fays, but to the intervals in the monochord that exprefs the feven notes in the diatonic scale of mufic. Laying hold of this analogy, which has little more than the fingle accidental coincidence abovementioned to fupport it, Father Caftel proceeded fo far as to contrive, and, if we mistake not, to construct a fingular machine, to which he gave the name of the Clavegin Oculaire, or the Ocular Harpfichard: expecting that the eyes of the bebolders would be gratified by the fucceffion, or the admixture, of the different colours in his prifmatic gamut; as the ears of an audience are regaled by the melody, or the harmonious combination, of the fuppofed correfpondent tones and femitones in the scale of Guido.

M. de Mairan has long ago fhewn the various defects in this fuppofed analogy between found and colours : but were it much more perfect than it has been fuppofed, it is fufficiently evident, even a priori, or without any trial, that we are not formed to receive pleafure from coloured thirds and fifths, in any respect, or degree, refembling that which refults from a mixture or fucceffion of founds. Certain colours are undoubtedly pleafing

+ See Memoires de l'Acad. Roy. de Sciences de Paris, année 1737, page 34, Dutch edition.

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in themselves, or when they fucceed, or are combined, or contrafted, with each other. Figuratively too we speak of the barmony of the tints in a well-coloured picture: but no human being, we imagine, has his optic organs fo conftituted as to relish a tune played by Father Caftel's Ocular Harpsichord; or by making differently coloured pieces of paper pass before his eyes in fucceffion, prefented through apertures of different lengths made in a hollow revolving cylinder; although these lengths fhould refpectively and exactly correfpond with those of certain intervals of the monochord.

Dr. Hooper feems however to be of a different opinion from us in this particular: for after informing us that though M. Guyot (from whom his defcription of this extraordinary machine is taken) commends the good Father's ingenuity and induftry, yet that he attributes but little merit to his fcheme; he adds, that nevertheless it is evident that there is a much ftronger relation between founds and colours than M. Guyot feems to imagine.' But this ftronger relation the Author does not condefcend to point out.

In the remaining part of this volume the Author exhibits fome of the experiments founded on Acoustics, or the doctrine of founds, which he terminates with a defcription of the organ.

The first part of the third volume contains fome of the more amufing electrical experiments, principally collected from Dr. Priestley and others. In order,' the Author fays, to add furprize to learning and ingenuity, it will be neceffary to conceal the apparatus;' and he accordingly directs how the electric matter may be conveyed, from the prime conductor placed in an adjoining room, to a candle branch fixed, in the exhibition room, to a pannel of the wainscot that has been previously baked, and which may be occafionally taken out and dried, whenever its infulating power has been diminished by damp weather.

If electricity were a fecret in the hands of a few adepts, the moft lucrative employment in the world perhaps would be that of an electrical conjurer, poffeffed of an apparatus thus kept out of fight. But as matters are now circumftanced, this expedient to give a magical air to electrical phenomena is puerile, and can scarce any where in Europe anfwer the purpose for which it is calculated. Electrical appearances are of so very diftinguishable a nature, and are fo generally known, that the moft ignorant spectator would be able to detect the philosophical conjuror almoft at his very firft fetting off. And with refpect to thofe who are of a philofophical turn, the phenomena of electricity are just as well qualified to excite astonishment, when the puny apparatus that produces them is in fight, as when it is concealed.

One of the Author's propofed electrical recreations appears to us to be rather of too serious a nature to deserve that title; and we shall on that account take notice of it.

In order to diverfify the experiment of giving the electric shock to a number of perfons, which is ufually performed by making them join hands, the Author obferves that the recreation may be agreeably varied by directing them to lay their hands on each other's heads; adding, with the greatest fangfroid imaginable, that if the whole company should be truck to the ground, as it happened when Dr. Franklin once gave the fhock in this manner to fix ftout men, the inconvenience will be very little for that the Doctor's patients immediately got up again without knowing what had happened.

This recreation may be very delectable to the byestanders, who are out of the circle, and in the fecret, but may certainly be attended with difagreeable, if not ferious, confequences to the performers in it; fome of whom ftand a fair chance of rifing with broken elbows, at least, in confequence of being fuddenly felled to the ground, fenfelefs, and totally unprovided for a fall; while others may acquire an inveterate headach, or other permanent injury, by this practical joke, which ought therefore to be expunged out of the lift of philofophical recreations.

The fucceeding and largest part of this volume teems with magic and conjuration, performed by means of an invisible and more filent agent than the electric matter: we mean the Magnet, on the attractive, repellent, and directive qualities of which are founded many fingular tricks and exhibitions; which, together with the machinery by which they are produced, are here minutely described and delineated. Some of thefe by being combined with other manoeuvres, are well adapted to excite furprile, even in those who know or fufpect that magnetism is concerned in the deception.

The experiments or recreations defcribed in the fourth volume relate to Pneumatics, Hydrology, and Pyrotechnics. These are followed by others reducible to the clafs of Chemistry; and the work is terminated by various recreations of address and dexterity,' principally performed with cards; which, as we have before obferved, furnish no inconfiderable part of the entertainment prefented in the preceding volumes.

As many of the experiments contained in this work are well known; and as feveral of thofe which are more fingular are very complex, and require an apparatus that cannot be intelligibly defcribed without the ufe of plates; we fhall give the fubftance of two or three amufing experiments, that are of a more fimple nature, as further fpecimens of this collection.

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