STATICS. by this means, we may find, not only the difference in weight, between the juices of plants of differing kinds, but, on fome occafions, obferve how far the keeping of a juice, for fome time, or the fermentation, or putrefaction thereof, will alter its fpecific gravity. Other liquids, alfo, that are used in phyfic, but not ponderable in water, may, by this way, be examin'd; as honey, vinegar, verjuice, &c. And, by the fame method, likewise, the fpecific gravity of the juices of fruits, may be difcover'd, and compared together. And hence a perfon of curiofity will, probably, be enabled to take notice of the differences produced in their specific gravities, thro' their feveral fucceffive ftates, at different times. Thus the juice of ripe grapes alters when newly prefs'd; again, when it begins to ferment; when it becomes new wine; when it has attain'd its full maturity, and perfection and, lastly, when it degenerates into prick'd wine, and abfolutely changes into vinegar, or Vappa.

All waters near

weight.

It may be here proper to obviate an objection, that will, probably, be ly of the fame made against the method hitherto deliver'd, of finding a proportion in weight, betwixt a finking body, and water of the fame bulk. Naturalifts might alledge, that, by this method, we cannot difcover the proportion between a folid body, and water in general; but only betwixt the propofed body, and the particular water 'tis weigh'd in; because there may be a great disparity between liquors that are call'd common water. Thus fome travellers tell us, that the water of the river Ganges is, by a fifth part, lighter than ours.

To this plaufible objection, I answer, firft, that having had opportunity to examine the weight of various waters, fome of them taken up in places very diftant from one another, I found the difference between their fpecific gravities to be exceeding fmall; perhaps, not above the thousandth part of the weight of either: nor did I find any confiderable difference, between the weight of feveral waters of differing kinds; as between fpringwater, river-water, rain-water, and fnow-water: tho' this laft was fomewhat lighter than any of the reft. And having received water from the river Ganges itself, I found it very little, if at all, lighter than fome of

our common waters.

And, fecondly, 'tis not neceffary, that the proportion obtainable by our method, fhould be mathematically exact: for, in experiments, where we are to deal with grofs matter, and employ material inftruments, a phyfical accuracy will fuffice.

To difcover the There is an ufe of hydroftatics, which tho' it do not directly tend to the magnitudes of bodes, hydrofta- examination of the Materia medica, may yet be ferviceable, both to the tieally. physician and the naturalift, in delivering defcriptions thereof; and thereby indirectly conduce to the knowledge of drugs; and help to diftinguish between genuine and adulterate fimples. 'Tis known, that the writers upon the Materia medica, ufually fet down the magnitude of the bodies they defcribe, by very uncertain gueffes; and thofe who affign them determinate meafures, either do it by faying, fuch a fruit, for example, is an inch, or two inches long, and half an inch, or a whole one, broad: but, accor

ding to this way of defcribing bodies, there may, by reafon of the great STATICS. variety of figures they are capable of, be a very great difference in the bulk of bodies, to each of which, the fame length and breadth are applicable.

I caused several cubes to be carefully made, by skilful artificers, of different fizes, and materials, whofe fides were each of them an exact inch, or a precife number of inches. These cubes, being carefully weigh'd in exact balances, firft in air, and then in common water, we concluded, from the refult of our feveral trials, that, without confiderable error, a cubical inch of water might be fuppofed to weigh 256 grains.

Suppofe, then, for example, that a folid heavier in fpecie than water, having been weigh'd firft in the air, be found in water to lofe 256 grains of its weight; the dimenfions of this folid, if it were of a cubical fhape, will equal a cubic inch; fo that if the given body be fuppos'd a metal eafily fufible, as tin, or lead, melted and pour'd into a hellow, metalline, cubic inch, and fuffer'd to cool, it would exactly fill it. For a finking folid weighs lefs in water than in air, by the weight of an equal bulk of water. And, as a cubical inch of water weighs 256 grains, it follows, that when the decrease of a body's weight in water, is 256 grains, the folid content of that body is a cubical inch; fince a body of water, weighing 256 grains, is equal in magnitude, as well to the folid, as to a cubic inch ofwater.

And here it may prevent a fcruple, to obferve, that to make bodies equal in magnitude, it is not neceflary that they fhould be of the fame weight, or matter; as is evident in bullets of copper, tin, and gold, caft in the fame mould. For, tho' they be all equal in bulk, yet the bullet of copper will be much heavier than that of tin; and the bullet of pure gold, more than twice as heavy as that of copper. Whenfcever, therefore, a folid, ponderous enough to fink in water, lofes therein 256 grains of the weight it had in air; the magnitude, or bulk of that body, is equal to a cubical inch; of whatever matter it confifts, or, of what fhape foever it be. And, in cafe the folid propofed, lofes of its weight in the water, lefs than 256 grains; its bulk will be proportionably iefs than a cubical inch. Thus every 32 grains that the folid lofes of its weight in the water, anfwers to an eighth of an inch in the bulk of the body: fo that if the decrease be 128 grains, the folid will be half a cubic inch; and if but 64 grains, a quarterof a cubic inch: on the other fide, if the decrease of the given body, exceed the ftandard, 256 grains, twice, thrice, &c. the body will be equal to two, three, &c. cubical inches.

'Tis eafy, from the doctrine deliver'd, to difcover, hydroftatically, the To gain the fofolid contents of a body, heavier in fpecie than water: but to meafure, by lidity of a body, the help of water, the folidity of a body lighter in fpecie than that fluid, hydrostaticaly, is a work of more difficulty. In order to it, we may confider there are water. two forts of bodies, which will not, naturally, fink in water; fome, being of a clofer texture, refifting it; and others abounding with pores, which difpofe them to imbibe the water.

X x 2

Now

STATICS.

Now the weight of a body of the firft kind, may be gather'd from that of the water equal in magnitude to the immerfed part of the body, when it floats freely thereon. Thus, if a parallelopipid, or a cylinder of wood twelve inches long, placed upon water, fhould reft there when a twelfth part of it lies beneath the furface of the fluid; the weight of the water, equal in bulk to that immerfed twelfth part, would be equal in weight to the whole body of the wood. But because the bodies, whofe bulk phyficians and chymifts may have occafion to examine, will very feldom happen to have regular figures; 'tis proper to add another method, more fuitable to the prefent defign. To measure then the folid contents of bodies fpecifically lighter than water, but irregularly fhaped; the body muft, firft, be weigh'd in air. 2. A plate of lead, capable of finking this body in water, and of fome round fum in weight, fhould be provided. 3. This plate being weigh'd in water, and its weight therein fubftracted from its weight in air, will give the weight of as much water, as is equal in bulk to the immerfed lead; or the fpecific weight of the lead in water. 4. The plate of lead, and the lighter body, must be tied together with horfe-hair, and the weight of the aggregate noted. 5. This aggregate must be weigh'd in water, and its weight therein fubftracted from that it had in air; and the difference will give the fpecific weight of the faid aggregate in water. 6. From this difference fubftract the fpecific weight of the plate in water, and the remainder will give the weight of the lighter body in the fame fluid. Then, laftly, that weight of the light body in water being divided by 256 grains, will give its folid content.

To clear and confirm this method by an example; we took a conveniently shaped piece of oak, that weigh'd in air 193grains; to this we tied, with an horfe-hair, a plate of lead, weighing juft 240 grains. But, before we tied them together, the lead was weigh'd in water, where it loft of its former weight 20 grains; which, being deducted out of the 240 grains, left a difference of 20 grains, for the fpecific weight of the lead in the water. Then the aggregate of the wood and lead was weigh'd, firft in the air, and found to be 433 grains, and next in water, where it amounted but to 162 grains; which being fubftracted from the aggregate of the fame bodies in air, the difference was 271 grains; from which, the other difference of 20 grains of the leaden-plate in water being deducted, there remain'd 251 grains, for the weight of a bulk of water equal to that of the given piece of wood. Now, if this number of grains had amounted to 256, we might have concluded the folidity of the body

*There is a very cafy method of obtaining the content, or cubic measure, of any folid, tho' ever fo irregularly figured. For, fince fuch a body, immerfed in a fluid wherein it will not diffolve, caufes that fluid to rife in direct proportion to the bulk immerfed; if the fluid be conFain'd in a cylindrical or prifmatic veffel,

and the additional height given it by the body be marked on the veffel; this will enable us to measure a part of the whole cylinder or prifm, equal in bulk to the irregular body. And thus may the folid content of ftatues, carv'd or emboss'd work, &c. be known.

to

to be a cubic inch; fince 256 grains of water, which we formerly found STATICS. equal to a cubic inch of water, was alfo now found equal to the bulk of the given piece of wood. And having caufed the wood I employ'd to be formed into a cubic inch; the difference of its weight in water from 256 grains, may, probably, be imputed to fome little imperfection in the figure, or other like circumftance. Firft, then, the cube of oak in air, weigh'd 193 grains. 2. The lead in air, 240 grains. 3. The lead in water, 220 grains; which, fubftracted from its weight in air, leaves for its specific weight in water, 20 grains. 4. The aggregate of the two in air, 433 grains. 5. The weight of both together in water, 162 grains; which, fubftracted from the fum of their weight in air, gives the difference of the two feveral aggregates, 271 grains. 6. The difference between the weight of the lead in air, and in water, 20 grains; which, fubftracted from the difference of the weights of the aggregates in air, and in water, gives for the weight of the propofed cube 251 grains.

This way of meafuring bodies is appropriated to fuch of them as will not readily diffolve in water. But, because there are many other folids, as falt, alum, vitriol, fugar, &c. whofe magnitudes it may often be proper to know, and compare; I fhall add, that the fame method is applicable, alfo, to folids diffoluble in water, if, inftead of that fluid, be fubftituted oil of turpentine, whofe proportion of fpecific gravity to water, is otherwife known. To difcover this, I employ'd the hollow cubic inch of brass, made use of to find the weight of a cubic inch of water; and found, that, when carefully filled, it contained 221 grains of this oil: by which number, the difference of the weight of a folid in the air, and in that oil, being divided, the quotient will give the folid contents of the body to be examined.

Let us now confider how fuch bodies as, by their porofity, are fubject to imbibe too much of the liquor, while the experiment is in hand.

Merfennus's expedient, in this cafe, is to cover the body to be weigh'd in water, with wax, pitch, or fome other glue, of a known fpecific weight in water. But I prefer bees-wax for this purpofe; and proceed with it in the following manner. The folid which is lighter than water, having been firft weigh'd in air, over-lay it carefully with thin bees-wax; then take, alfo, in the air, the weight of the wax employ'd; and faften to the body thus coated, a plate of lead, or tin, heavy enough to fink it; and obferve the weight of the aggregate in water. This done, fubftract the weight of as much water as is equal in bulk to the wax, and proceed as is before taught.

But this method helps us only to the weight of the propofed body in water; to discover its folid content, we muft divide the weight of the folid in water, by 256 grains.

And, by the way, I have, fometimes, made ufe of another expedient to hinder fmall folids, whether lighter or heavier in fpecie than water, from imbibing the liquor wherein they were weigh'd: for having first found the weight of a cubic inch of quick-filver; we placed the body to be mea

fured,

STATICS. fured, in a veffel, whofe folid contents were known before-hand; when, the space that remain'd unpoffeffed by the firm body being fill'd with quickfilver, 'twas eafy to know, by the difference in weight of that quickfilver, from the weight of the quick-filver requifite to fill the whole veffel, how much thereof was equal to the furrounded body. And, by this means, and the knowledge before gain'd of the weight of a cubical inch of mercury, the folid content of the body propofed, was easily obtain❜d.

What accuracy is to be expected

experiments.

Before I put an end to this difcourfe, 'tis proper to fhew what credit in bydropatical may be given to the eftimates of the weights and proportions of bodies obtain'd by hydroftatical trials; becaufe mathematicians, either not knowing, or not applying our obfervation about the fpecific gravity of rockcryftal, and the nature of oil of turpentine, have given us very different accounts of the proportions of metals, and a very few other familiar bodies, which are all they feem'd to have examin'd by this method.

And, indeed, I fhould not be furprized to find, that the experiments of the fame perfon, made at diftant times, and under different circumftances, difagreed; for fome difference there may be betwixt the waters employ'd in this cafe; efpecially if the air be at one time intenfely hot, and at another exceeding cold. The difference, alfo, of degrees in goodness of the balances used in nice experiments, is not altogether inconfiderable. But a greater hindrance to the accuracy of hydroftatical experiments, is the difficulty of finding an exact uniformity in weights of the fame denomination, which are vulgarly fuppofed to be exactly equal. I have myself found it fo difficult in practice to procure, and keep weights as exact as I defired, that I have left off the hopes of it; for the very air may, in time, caufe an alteration in them. And tho' the accurate Ghetaldus's tables of the weight of metals, and fome other bodies, in refpect to one another, are look'd upon as the most authentic that have been publifh'd; and are, accordingly, the moft made ufe of; yet 'tis certain, the weights he employ'd are not divided as ours are. For tho', according to him, as well as with us, the ounce confifts of four and twenty fcruples; yet the fcruple, which with us is divided but into twenty grains, he divides into twenty-four. But tho' hydrostatical experiments are not, always, either fingly accurate, or exactly agreeable to one another; they prove accurate enough to be very useful in practice, and more exact than any other method, hitherto employ'd, of determining the proportions of bodies, in point of weight and bulk, and of measuring their folid contents; but, efpecially, fuch little ones as are neceffary to be examined in the Materia medica. And this is a corollary from the whole of what we have deliver❜d.

;

And, indeed, as little as my skill is in hydroftatics, I would not be debarr'd from the use of it, for a very valuable confideration; for it has already done me acceptable fervice, and on a vaft variety of occafions efpecially in the examination of metals and mineral bodies, and of feveral chymical productions. I have often been able, by its means, to undeceive artifts in their perfuafion of poffeffing Luna fixa, and other valuable commodities; and to make a judgment, as to the genuineness

or