Part 76 (2/2)
_To shew the Spots in the Sun's Disk, by its image in the Camera Obscura._
Put the object-gla.s.s of a ten or twelve feet telescope into the scioptric ball, and turn it about till it be directly opposite the sun: when the sun is directly opposite the hole, the lens will itself be sufficient; or by means of the mirror on the outside of the window, as in the last recreation, in the focus of the lens, and you will see a clear bright image of the sun, of about an inch diameter, in which the spots on the sun's surface will be exactly described.
As this image is too bright to be seen with pleasure by the naked eye, you may view it through a lens, whose focus is six or eight inches diameter, which, at the same time that it prevents the light from being offensive, will, by magnifying both the image and the spots, make them appear to greater advantage.
_To magnify small Objects by means of the Sun's Rays let into a dark Chamber._
Let the rays of light that pa.s.s through the lens in the shutter be thrown on a large concave mirror, properly fixed in a frame; then take a slip, or thin plate of gla.s.s, and sticking any small object on it, hold it in the incident rays, at a little more than the focal distance from the mirror, and you will see, on the opposite wall, amidst the reflected rays, the image of that object, very large, and extremely clear and bright. This experiment never fails to give the spectator the highest satisfaction.
_To cut a Looking-gla.s.s, or piece of Crystal, let it be ever so thick, without the help of a Diamond, in the same shape as the Mark of the Drawing made on it with Ink._
This remarkable operation unites utility with amus.e.m.e.nt; for being in the country, or in a place where there is no glazier to be had, the following means will answer the purpose without their help.
Take a bit of walnut-tree, about the thickness of a candle, and cut one of its ends to a point; put that end in the fire, and let it burn till it is quite red: while the stick is burning, draw on the gla.s.s or crystal, with ink, the design or outline of the form in which you mean to cut it out: then take a file, or bit of gla.s.s, and scratch a little the place where you mean to begin your section; then take the wood red-hot from the fire, and lay the point of it about the twentieth part of an inch, or thickness of a guinea, from the marked place, taking care to blow always on that point, in order to keep it red; following the drawing traced on the gla.s.s, leaving, as before, about the twentieth part of an inch interval every time that you present your piece of wood, which you must take care to blow often.
After having followed exactly the outlines of your drawing, to separate the two pieces thus cut, you need only pull them up and down, and they will divide.
_By the means of two plain Looking-gla.s.ses, to make a Face appear under different forms._
Having placed one of the two gla.s.ses horizontally, raise the other to about right angles over the first; and while the two gla.s.ses continue in this posture, if you come up to the perpendicular gla.s.s, you will set your face quite deformed and imperfect; for it will appear without forehead, eyes, nose, or ears, and nothing will be seen but a mouth and a chin boldly raised: do but incline the gla.s.s ever so little from the perpendicular, and your face will appear with all its parts, excepting the eyes and the forehead; stoop a little more, and you will see two noses and four eyes; and then a little further, and you will see three noses and six eyes;--continue to incline it still a little more, and you will see nothing but two noses, two mouths, and two chins; and then a little further again, and you will see one nose and one mouth; at last incline a little further, that is, till the angle of inclination comes to be 44 degrees, and your face will quite disappear.
If you incline the two gla.s.ses, the one towards the other, you will see your face perfect and entire; and by the different inclinations, you will see the representation of your face, upright and inverted, alternately.
_To know which of two different Waters is the lightest, without any Scales._
Take a solid body, the specific gravity of which is less than that of water, deal, or fir-wood, for instance, and put it into each of the two waters, and rest a.s.sured that it will sink deeper in the lighter than in the heavier water; and so, by observing the difference of the sinking, you will know which is the lightest water, and consequently the wholesomest for drinking.
_To know if a suspicious Piece of Money is good or bad._
If it be a piece of silver that is not very thick, as a crown, or half a crown, the goodness of which you want to try; take another piece of good silver, of equal balance with it, and tie both pieces with thread or horse hair to the scales of an exact balance, (to avoid the wetting of the scales themselves,) and dip the two pieces thus tied, in water; for then, if they are of equal goodness, that is, of equal purity, they will hang in equilibrio in the water as well as in the air: but if the piece in question is lighter in the water than the other, it is certainly false, that is, there is some other metal mixed with it, that has less specific gravity than silver, such as copper; if it is heavier than the other, it is likewise bad, as being mixed with a metal of greater specific gravity than silver, such as lead.
If the piece proposed is very thick, such as that crown of gold which Hiero, king of Syracuse, sent to Archimedes, to know if the goldsmith had put into it all the eighteen pounds of gold that he had given him for that end; take a piece of pure gold of equal weight with the crown proposed, viz. eighteen pounds; and without taking the trouble of weighing them in water, put them into a vessel full of water, one after another, and that which drives out most water, must necessarily be mixed with another metal of less specific gravity than gold, as taking up more s.p.a.ce, though of equal weight.
_To hold a Gla.s.s full of Water with the Mouth downwards, so that the Water shall not run out._
Take a gla.s.s full of water, cover it with a cup that is a little hollow, inverting the cup upon the gla.s.s; hold the cup firm in this position with one hand, and the gla.s.s with the other; then with a jerk turn the gla.s.s and the cup upside down, and so the cup will stand upright, and the gla.s.s will be inverted, resting its mouth upon the interior bottom of the cup: this done, you will find that part of the water contained in the gla.s.s will run out by the void s.p.a.ce between the bottom of the cup, and the brim of the gla.s.s; and when that s.p.a.ce is filled, so that the water in it reaches the brim of the gla.s.s, all pa.s.sage being then denied to the air, so that it cannot enter the gla.s.s, nor succeed in the room of the water, the water remaining in the gla.s.s will not fall lower, but continue suspended in the gla.s.s.
If you would have a little more water descend into the cup, you must, with a pipe or otherwise, draw the water out of the cup, to give pa.s.sage to the air in the gla.s.s; upon which, part of the water will fall into the gla.s.s till it has stopped up the pa.s.sage of the air afresh, in which case no more will come down; or, without sucking out the water in the cup, you may incline the cup and gla.s.s so that the water in the cup shall quit one side of the brim of the gla.s.s, and so give pa.s.sage to the air, which will then suffer the water in the gla.s.s to descend till the pa.s.sage is stopped again.
This may likewise be resolved by covering the brim of the gla.s.s that is full of water, with a leaf of strong paper, and then turn the gla.s.s as above; and without holding your hand any longer upon the paper, you will find it as it were glued for some time to the brim of the gla.s.s, and during that time the water will be kept in the gla.s.s.
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