Imágenes de páginas

<more you

e !!ght matter,

• , • fuch as down, or the downy parts of feathers, their speci<fic gravity is much lessened; and that by holding another

electrified body under them, they may be driven upwards at pleasure. It is also evident, from experiment, that the

divide the parts of such bodies, the more of their <specific gravity they will lose by being electrified, and by • dividing them into very minute parts, I have found, that they ascended to a considerable height after they were electrified. From whence I think it highly probable, that the

exceeding small particles of vapour and exhalation may be, • and are fufficiently electrified to render them specifically

lighter than the lower air; and that they do afcend by that means, And that they will ascend proportionably higher, € as the surrounding fluid is proportionally greater than the

particle which is carried up.'

Our Author next proceeds to fhew, that the afcent ånd descent of vapour and exhalation, attended by this electrical fire or Auid, is the cause of all the regular and irregular motions we find in the atmosphere. And also, First, • why it generally rains in winter while the wind is south, • fouth-west, and wefterly. Secondly, why north-west winds

are generally attended by showers in the beginning, and become more dry as they are of longer continuance. Thirdly, why north and north-east winds are generally dry. Fourthly, why the east wind continues dry and dark for a confia derable time together. Fifthly, why squalls precede heavy and diftinct showers; and why a calm ensues for fome little

time after they are passed. Sixthly, why ftorms and high (winds feldom happen in a serene sky, without clouds. Se

venthly, why the vapours, in warm feasons, coalesce <form those distinct dense clouds, which produce thunder and

heavy showers. Eighthly, why the barometer falls loweft in

long continued rains, attended by winds; and why it rises • highest in long continued fair weather; and why the inter

mediate changes happen. Ninthly, of 'land-breezes, and fea-breezés, and water-spouts."

It would extend this article too far, to fhew in what manner Mr. Eeles has accounted for these phænomena; we thall therefore only add the experiments by which our Author found all afcending vapours and exhalations to be electrified.

• I extended,' says he, “a fine string of filk eight feet horizontally, and from the middle suspended two pieces of such down as grows upon our turf-bogs, by two pieces of fine filk, about twelve inches each in length; and then, by rub

[ocr errors]

Telec their nature, the aforementioned experiments would have fies the down, and fo96

bing a piece of sealing-wax on my waistcoat, over my side, I • electrified the pieces of down; and then brought sundry burning things under them, so as to let the smoke pals in great

plenty thro' and about them, to try whether the electric fluid • would run off with the smoke; but I had the pleasure to fee,

that the down was but a little affected by the passage of the < smoke, and still remained electrified. I then brought fundry ofteams from the spout of a boiling tea-kettle, and otherwise, in • trified. I then breathed on them in great plenty, but found

that the down still remained electrified. I then joined the • palms of my hands together, with the fingers extended per

pendicularly under the down, which still remained electrified, < altho' the fubtile efluvia, thrown off by perspiration, passed

in great plenty through the down; as may appear by hold

ing one or both the hands in the same manner, under any • light matter floating in the air, which will be driven upTwards thereby, with a great velocity as an electrified feather " is, by any electrified body held under it. In short, I tried

all the vapours and exhalations I could think of, in the same • manner, and with the same success.

I then warmed a wine-glass, and with the skirt of my coat held inside and outside, the glass between my fingers

and thumb: I rubbed the glass briskly about, and electrified • the down, and found all experiments answer in the same • manner as they did with the wax. I mention this particular « because fome writers on electricity have said, that there were «two kinds of electrical fire, the one resinous, and the other • Vitreous; because light bodies, electrified by glass, are at• tracted by electrified wax, &c. and those electrified by resins rare attracted by glass. But I think these different effects • můst arise from some differing qualities in the resin and glass, which have power to actuate this fire differently. For if there were really two distinct species of this fire, opposite in a very different consequence from what appears. For if the

vapours were imprégnated by the vitreous fire, they must ab< forb, or some way disturb the resinous fire, which electrifame vapour, with its electric fire, passes through the electrified down, in the same manner, whether it be electrified by glafs or refin.

The electricity remaining in the electrified down, after • these experiments made it appear, that the smoke and steams

must be either electrics, or non-electrics electrified, it was easy to suppose them non-electrics, as they arise from non3


[ocr errors]

electric bodies; and the more, because the highest electrics, by a discontinuity and comminution of their parts (long be

fore they come to be as minute as the particles of ascending • vapour) become non-electrics, or conductors of electricity: • For glass, resin, wax, &c. all become non-electric, even

in fusion. But to try whether the steams, &c. were non

electrics, I only bedewed the wax and glass with my breath, * fteams, &c. from my hand, to the end of the wax and < glass; and then touching the electrified down with the end • of the glass or wax, I found that the electrical fire imme

diately passed from the down into my hand, through the « steams, &c. which rested on the wax and glass. Which I • think fufficiently proves the fteams, &c. to be non-electric; ' and I think, that it as plainly appears, that they are all elec+ trified while ascending, because the electrical fire in the down

does not join with them in their paffage through it ; which

otherwise it would do with them, or any non-electric not 6 electrified.'

Art. 27. The Abbè Sauvages, of the Royal Society of Montpelier having discovered, that the juice of the Toxicodens dron Carolinianum foliis pinnatis, floribus minimis herbaceis tinged linen with a deeper black than any other known preparation, and without the least acrimony; the Abbè Mazeas, F.R.S. informs Dr. Hales, that it indeed is an excellent black, but that the juice of the Toxicodendron triphyllum folio finuato pubescente, T. 611. which is a native of Virginia, ftruck a finer and speedier black than the former, which however was exceeded by the juice of the Toxicodendron triphyllum glabrum, T. 611. Neither boiling water with foap, nor a strong lye of the ashes of green wood, diminished in the least the depth and splendor of these colours.

As the blacks of our painted cloths, prepared from iron and nut-galls, grow rusty after a certain number of washings, and in time wear out, this American, and more lasting, varnith, may be an improvement. Mr. Philip Miller, however, in. forms the society, that it is no discovery of the Abbè's, as Kæmpfer mentions the black-tinging quality of the Carolina Toxicodendron; and we know, that the Japanese stain all their utensils, and that the Calicuts paint themselves, with it --This varnish is obtained by wounding the tree (a). It is white, and clammy, at first, but soon turns black when exposed to the air; and needs no preparatioit, unless some dirt fhould mix with it, and

(a) When the shrub is sufficiently drained of its juices, they cut it down to the ground: New ftems arise from the root, which is three years are again fit for wounding.


then the Japanese strain it through a coarse gause, and putting it into wooden vessels, doubly secure it from evaporating, with oil, and a skin over it. A good varnith is also made of the juice of the Cathew-nut-tree.

The first species, the inhabitants of Carolina, and the Ba. hama islands, call poison-tree, and poison-ash; and the other two forts are named poison-oak, in Virginia and New England." These appellations are expreslive of their properties; for the varnish emits à poisonous vapour, which occasions vibient" head-achs, and swellings of the lips, of those who handle it, unless they tie a handkerchief over their nose and mouth.

As the Toxicodendrons are common in our northern, and the Cathew-nut-trees in our southern colonies of America; it were to be wished, concludes Mr. Miller, that the inhabitants of both would make some expériments to collect this várnih, as it may not only produce much profit to themfelves, but also become a national advantage. Art. 28. A Letter to the Right Hon, the Earl of Macclesfields

President of the Royal Society, concerning the method of conAtructing a table for the Probabilities of Life, at London; from the Rev. William Brackenridge, D. D. and F. R. S.

The generally assumed data, employed by political arith-" meticians in their calculations, have, when separately taken, been often found uncertain, and sometimes manifestly erroneous; nevertheless, that, in conjunction, there is more than a probability of making some useful deductions from them, this article pretty clearly evinces. The number of diffenters, of various denominations, as well forcigners as natives, of whose baptisms no account is taken, Dr. Brackenridge juftly obferves, renders the London bills of mortality very incompetent regifters of births : he also takes notice that, in computing the probability of life, there is no better dependence on the numbers of burials therein delivered, eipecially above the age of twenty; as about that period there is a continual acceflion of multitudes of strangers to this metropolis, on different occasions: whereby our burials are always in a fluctuating state. Hence he concludes, that those who have formed their calculations upon the London bills only, have been led into several mistakes and inconsistencies.

Dr. Halley, whose table has ever been esteemed the most exact, and useful, of any thing of this kind, founded his comi* putations on the bills of mortality at Breslau; but the differ ence of country, and way of living, having been objected

REVIEW, 04. 1756. Bb


against Dr. Halley's scheme, with respect to London, Dr. Brackenridge proposes to rectify the errors that have arisen from the separate confideration of these bills, by making use of ours, so far as the age of twenty years, and those of Breslau for the subsequent periods of life.

Upon this principle our learned calculator endeavours to determine the annual number of births here; in order to which, says he, we must have the number of burials known, at least

in the several periods, till the 20th year; viz. under two, • between two and five, between five and ten, and between • ten and twenty. And it is evident, if we suppose no accef

fion of strangers, that the number of the living, in any one

year, will be equal to the difference between the births and • the sum of all the subsequent burials at each age till that year.

The number of the living in any one year is easily known, • if we suppose the probability of life to be the same as at Bre« flau; for then the number of dead there, will be to the

number of the living, as the dead at London to the living. « Thus in the 20th year, the dead and living at Breslau are as + 6 and 598, and the dead at London are 73, or more exact• ly 72,88; therefore the living must be 7263. The dead

in the intermediate years at London may likewise be found, « by means of Dr. Halley's table. For, by proportion, if the · dead at Breslau, from the age of ten to twenty complete,

be 61, and in the 20th year 6, and the dead at London for the same period be 741 ; then will the dead in the 20th year be 73. And therefore if the living at London, in the 20th year of their age be found to be 7263; this must be equal to

the number of births, having substracted from them all the dead • in each of the preceding nineteen years. And consequently, <if we put * for the number of births, we shall have this • fimple equation: *--8819--2006-805-741 * +73=

7263; and thence the number of births X=19561. And • the same number would have been produced from any inter• mediate age, between twelve and twenty. So that, if we could ( be certain of the number of the dead, there could be no doubt, « but that 19561, would nearly, at an average for ten years, be

the whole of the births yearly. And this is greater than the “ number of baptisms known, 14626, taken likewise at a me

[ocr errors]

* Dr. Brackenridge has, in the beginning of the article, given a table of the number of burials, at the several ages, taken upon an average, from the bills of mortality, for ten years, from 1743 to 1753: to which he has annexed the numbers of the dead at the respective periods at Breilau. From this table these nnmbers are taken.

« AnteriorContinuar »