Part 76 (1/2)
Or, if you will, considering that gold is of a greater specific gravity than silver, the chest full of gold, though similar and of equal weight with the other, must needs contain a less bulk, and consequently it contains the gold.
_To find the Burden of a s.h.i.+p at Sea, or in a River._
It is a certain truth, that a s.h.i.+p will carry a weight equal to that of a quant.i.ty of water of the same bulk with itself; subtracting from it the weight of the iron about the s.h.i.+p, for the wood is of much the same weight with water; and so, if it were not for the iron, a s.h.i.+p might sail full of water.
The consequence of this is, that, however a s.h.i.+p be loaded, it will not totally sink, as long as the weight of its cargo is less than that of an equal bulk of water: now, to know this bulk or extent, you must measure the capacity or solidity of the s.h.i.+p, which we here suppose to be 1000 cubical feet, and multiply that by 73 pounds, the weight of a cubical foot of sea-water; then you have in the product 73,000 pounds for the weight of a bulk of water equal to that of the s.h.i.+p; so that in this example, we may call the burden of the s.h.i.+p 73,000 pounds, or 36-1/2 tons, reckoning a ton 2,000 pounds, that being the weight of a ton of sea-water; if the cargo of this s.h.i.+p exceeds 36-1/2 tons, she will sink; and if her loading is just 73,000 pounds, she will swim very deep in the water upon the very point of sinking; so that she cannot sail safe and easy, unless her loading be considerably short of 73,000 pounds weight; if the loading come near to 73,000 pounds, as being, for example, just 36 tons, she will swim at sea, but will sink when she comes into the mouth of a fresh water river; for this water being lighter than sea-water will be surmounted by the weight of the vessel, especially if that weight is greater than the weight of an equal bulk of the same water.
_To Measure the Depth of the Sea._
Tie a great weight to a very long cord, or rope, and let it fall into the sea till you find it can descend no further, which will happen when the weight touches the bottom of the sea: if the quant.i.ty or bulk of water, the room of which is taken up by the weight, and the rope, weighs less than the weight and rope themselves; for if they weigh more, the weight would cease to descend, though it did not touch the bottom of the sea.
Thus one may be deceived in measuring the length of a rope let down into the water, in order to determine the depth of the sea; and therefore, to prevent mistakes, you had best tie to the end of the same rope another weight heavier than the former, and if this weight does not sink the rope deeper than the other did, you may rest a.s.sured that the length of the rope is the true depth of the sea; if it does sink the rope deeper, you must tie a third weight, yet heavier, and so on, till you find two weights of unequal gravitation, that run just the same length of the rope, upon which you may conclude, that the length of the wet rope is certainly the same with the depth of the sea.
_Method of Melting Steel, and causing it to Liquefy._
Heat a piece of steel in the fire, almost to a state of fusion, then holding it with a pair of pincers or tongs, take in the other hand a stick of brimstone, and touch the piece of steel with it: immediately after the contact, you will see the steel melt and drop like a liquid.
_How to dispose two little Figures, so that one shall light a Candle, and the other put it out._
Take two little figures of wood or clay, or any other materials you please, only taking care that there is a little hole at the mouth of each: put in the mouth of one a few grains of bruised gunpowder, and a little bit of phosphorus in the mouth of the other, taking care that these preparations are made beforehand.
Then take a lighted wax candle, and present it to the mouth of the figure with the gunpowder, which, taking fire, will put the candle out; then present your candle, having the snuff still hot, to the other figure; it will immediately light again by means of the phosphorus.
You may propose the same effects to be produced by two figures drawn on a wall with a pencil or coal, by applying with a little starch, or water, a few grains of bruised gunpowder to the mouth of one, and a bit of phosphorus to the mouth of the other.
_The Camera Obscura, or Dark Chamber._
We shall here give a short description of this optical invention; for though it is very common, it is also very pleasing: but every one knows not how to construct it.
Make a circular hole in the shutter of a window, from whence there is a prospect of the fields, or any other object not too near: and in this hole place a convex gla.s.s, either double or single, whose focus is at the distance of five or six feet: the distance should not be less than three feet; if it be, the images will be too small, and there will not be sufficient room for the spectators to stand conveniently; on the other hand, the focus should never be more than fifteen or twenty feet, for then the images would be obscure, and the colouring faint; the best distance is from six to twelve feet:--take care that no light enters the room but by this gla.s.s: at a distance from it, equal to that of its focus, place a pasteboard, covered with the whitest paper; this paper should have a black border, to prevent any of the side rays from disturbing the picture; let it be two feet and a half long, and eighteen or twenty inches high; bend the length of it inwards to the form of part of a circle, whose diameter is equal to double the focal distance of the gla.s.s: then fix it on a frame of the same figure, and put it on a moveable foot, that it may be easily fixed at that exact distance from the gla.s.s where the objects paint themselves to the greatest perfection: when it is thus placed, all the objects that are in the front of the window will be painted on the paper in an inverted position; this inverted position of the images may be deemed an imperfection, but it is easily remedied; for if you stand above the board on which they are received, and look down on it, they will appear in their natural position; or if you stand before it, and, placing a common mirror against your breast in an oblique direction, look down in it, you will there see the images erect, and they will receive an additional l.u.s.tre from the reflection of the gla.s.s: or place two lenses in a tube that draws out: or, lastly, if you place a large concave mirror at a proper distance before the picture, it will appear before the mirror in the air, and in an erect position, with the greatest regularity, and in the most natural colours.
If you place a moveable mirror without the window, by turning it more or less, you will have on the paper all the objects that are on each side of the window.
There is another method of making the dark chamber, which is by a scioptric ball, that is, a ball of wood, through which a hole is made, in which hole a lens is fixed; this ball is placed in a wooden frame, in which it turns freely round: the frame is fixed to the hole in the shutter, and the ball by turning about answers, in great part, the use of the mirror on the outside of the window: if the hole in the window be no bigger than a pea, the objects will be represented without any lens.
If instead of placing the mirror without the window, you place it in the room, and above the hole, (which must then be made near the top of the shutter,) you may receive the representation on a paper placed horizontally on a table; and draw at your leisure all the objects that are there painted.
Nothing can be more pleasing than this recreation, especially when the objects are strongly enlightened by the sun; and not only land prospects, but a sea-port, when the water is somewhat agitated, or at the setting of the sun, presents a very delightful appearance.
This representation affords the most perfect model for painters, as well for the tone of colours, as that gradation of shades occasioned by the interposition of the air, which has been so justly expressed by some modern painters.
It is necessary that the paper have a circular form, for otherwise, when the centre of it was in the focus of the gla.s.s, the two sides would be beyond it, and consequently the images would be confused: if the frame were contrived of a spherical figure, and the gla.s.s were in its centre, the representation would be still more accurate. If the object without be at the distance of twice the focal length of the gla.s.s, the image in the room will be of the same magnitude with the object.
The lights, shades, and colours in the camera obscura, appear not only just, but, by the images being reduced to a smaller compa.s.s, much stronger than in nature; add to this, that these pictures exceed all others, by representing the motion of the several objects: thus we see the animals walk, run, or fly, the clouds float in the air, the leaves quiver, the waves roll, &c. and all in strict conformity to the laws of nature. The best situation for a dark chamber is directly north, and the best time of the day is noon.