Part 3

The form of a horse-shoe is generally given to magnetized bars, when both poles are wanted to act together, which frequently happens in various experiments, such as for lifting weights by the force of magnetic attraction, and for magnetizing steel bars by the process of double touch, for which they are exceedingly convenient. The following is the method of making a powerful magnetic battery of the horse-shoe form. Twelve bars or plates of steel are to be taken, and having been previously bent to the required form, that is, the horse-shoe shape, they are then bound together by means of rivets at their ends; before being finally fastened they are each separately magnetized and afterwards finally united.

Horse-shoe magnets should have a short bar of soft iron adapted to connect the two poles, and should never be laid by without such a piece of iron adhering to them. Bar magnets should be kept in pairs with their poles turned in contrary directions, and they should be kept from rust. Both kinds of magnets have their power not only preserved but increased, by keeping them surrounded with a mass of dry filings of soft iron, each particle of which will re-act by its induced magnetism upon the point of the magnet to which it adheres, and maintain in that point its primitive magnetic state.

EXPERIMENT TO SHOW THAT SOFT IRON POSSESSES MAGNETIC PROPERTIES WHILE IT REMAINS IN THE VICINITY OF A MAGNET.

Let a magnet and a key be held horizontally near one of its poles, or near its lower edge. Then if another piece of iron, such as a small nail, be applied to the other end of the key, the nail will hang from the key, and will continue to do so while the magnet is slowly withdrawn; but when it has been removed beyond a certain distance, the nail will drop from the key, because the magnetism induced in the key becomes at that distance too weak to support the weight of the nail. That this is the real cause of its falling off may be proved by taking a still lighter fragment of iron, such as a piece of very slender wire, and applying it to the key. The magnetism of the key will still be sufficiently strong to support the wire, though it cannot the nail, and it will continue to support it even when the magnet is yet further removed; at length, however, it drops off.

ELECTRO-MAGNETISM.

The identity of magnetism with electricity alluded to in a former paragraph, has led to the formation of a new science under the above name, and to some of the interesting experiments connected with it, we shall briefly allude for the amusement of the young reader.

POWER OF THE ELECTRO-MAGNET.

The same influence which affects the magnetic needle already described, will also communicate magnetism to soft iron. If a bar of that metal bent, be surrounded with a common bonnet wire, or a copper wire prevented from touching the iron by a winding of cotton or thread, and then if a current of voltaic electricity be sent through the wire, the bar becomes a powerful magnet, and will continue so as long as the connection with the battery is preserved. On breaking the contact, the magnetism disappears. This experiment may be easily made by the young reader with a horse-shoe magnet, surrounded by several coils of wire.

THE MARINER’S COMPASS AND EXPERIMENTS WITH A POCKET COMPASS.

The mariner’s compass is an artificial magnet fitted in a proper box, and consists of three parts--the box, the card or fly, and the needle. The box is suspended in a square wooden case, by means of two concentric brass circles called gimbals, so fixed by brazen axes to the two boxes, that the inner one, or compass-box, retains a horizontal position in all motions of the ship. The card is a circular piece of paper which is fastened upon the needle, and moves with it. The outer edge of the card is divided into thirty-two points, called points of the compass. The needle is a slender bar of hardened steel, having a hollow agate cup in the center, which moves upon the point of a pivot made of brass.

VARIATION OF THE NEEDLE.

The magnetic needle does not point exactly north and south, but the north pole of the needle takes a direction to the west of the true north. It is constantly changing, and varies at different parts of the earth, and at different times of the day.

DIP OF THE NEEDLE.

Another remarkable and evident manifestation of the influence of the magnetism of the earth upon the needle is the inclination or dip of the latter which is a deviation from its horizontal place in a downward direction in northern regions of its north, and in southern regions of its south pole. In balancing the needle on the card, on account of this dipping, a small weight or movable piece of brass is placed on one end of the needle, by the shifting of which either nearer to or further from the center, the needle will always be balanced.

USEFUL AMUSEMENT WITH THE POCKET COMPASS.

Pocket compasses are to be bought for from 50 cents to $1, and may be used in many ways. In traveling over mountains or a wide extended plain, they are indispensably necessary, and no one should go on a tour without such a companion; it will be a very useful and amusing exercise for any young person to take the bearings of his own or some particular locality, and make out what may be called a bearing card. This he may easily do in the following manner: Supposing he wishes, for instance, to take the bearings of his own house, he has nothing to do but set his pocket compass upon a map of the district,--a county map will do very well, unless his house stands on the verge of a county, then two county maps will be necessary. He must make the north of the map exactly coincide with the north, as indicated by his compass, and having fixed his map in this situation, he should take a ruler and piece of paper, and dot down the exact bearings of each important town, or place, or village, around him. Let him suppose himself, for instance, in the town of Albany, N. Y., and laying down his map as indicated by the compass, north to north and south to south, he will find the following places due north, Balston Spa; Hudson, south; Schoharie, west. The other points of the compass may be filled up in the same manner. Should, therefore, our young friend be upon any other elevated situation near his own dwelling, or upon any other elevated spot from which the bearings have been taken, he will be able to inform his young friends that such and such a place lies in such a direction, that this place lies due north, the other north-west, a third south-east, the fourth south-west, etc., etc.

INTERESTING PARTICULARS CONCERNING THE MAGNET.

Fire-irons which have rested in an upright position in a room during the summer months are often highly magnetic.

Iron bars standing erect, such as the gratings of a prison cell, or the iron railings before houses, are often magnetic.

Great iron-clad ships are powerfully magnetic, and therefore affect the compass by which the vessel is steered; ingenious arrangements are therefore made to correct the effect of the local attraction, so that the man-of-war may be steered correctly.

Magnetism may be made to pass through a deal board; to exhibit which, lay a needle on the smooth part above, and run a magnet along the under side, and the needle will be found to follow the course of the magnet. A magnet dipped into boiling water loses part of its magnetism, which, however, returns upon its cooling.

A sudden blow given to a magnet often destroys its magnetic power.

HOW TO BECOME A PHOTOGRAPHER.

Associated with the use of iodine and bromine is an art which every intelligent boy may practice, if he will attend to the following precise details kindly furnished by an experienced photographer.

HOW TO MAKE THE NEGATIVE ON GLASS, USING COLLODION BROMOIODIZED FOR IRON DEVELOPMENT.

1. The edges of the glass should be ground all round, also slightly on the surface of the edges. This prevents contraction of the film, enabling it to resist the action of a heavy stream of water. Mark one side in the corner with a diamond, and upon this side bestow the greatest care.

2. _To clean the glass, if new._--Make a mixture of spirits of wine and solution of ammonia, equal parts; render it as thick as cream with tripoli; with a piece of cotton-wool kept for this purpose rub a small quantity over that side marked as described, wash well under a tap of water, and wipe dry with a piece of old linen, washed without soap, and kept scrupulously clean for this purpose. Plates should not, however, be cleaned in the operating room with the above mixture; the vapor of ammonia might prove injurious to the chemicals.

3. Now polish with an old white silk handkerchief. If this latter precaution be not taken, small particles of linen will be left upon the plate: these are perhaps only seen when draining off the collodion; they form nuclei and eddies, checking the collodion in its course. Some of these minute fibers are washed off, and contaminate the next picture. To all lovers of clean pictures our advice therefore is, having well dried the plate with old linen, lay it, clean side upwards, upon a few sheets of common glazed demy paper (not blotting), and rub it hard with the silk until sensibly warm; this has the double advantage of dispersing fibers and moisture, for all glass plates are slightly in a hygrometric condition. Double the silk rubber up to form a pad, and with this the glass must be firmly dusted down just before pouring on the collodion, which will then run most evenly; if the coated plate is now viewed by transmitted light, not a speck or blemish will be seen upon it. When a plate cleaned as above described is breathed upon, the moisture does not evaporate slowly, but _flies off_. Do not be afraid of putting the glass into an electrical condition with the silk rubber; on this account objections have been raised to the use of silk; practically, however, I find it a most valuable auxiliary in this starting-point of the process, the perfect manipulation of which makes an important difference in the value of the finished picture. What can be more inartistic and annoying to an educated eye than spots, patches, stars, and sky-rockets, the forms and shapes of which rival, in numberless variety, a display of fireworks? Let us not, therefore, be contented with pictures, however good in other respects, presenting these deformities--so many blots on the photographic escutcheon.

_To clean a glass after having used it, when not varnished._--Wash off the collodion film with water, then clean the marked side with plain tripoli and water, and dry as above.

_To coat the plate._--First remove all the particles of dried collodion from the mouth of the bottle. Now pour upon the center of the cleaned glass as much collodion as it will hold. Do not perform this operation hurriedly, take time, and systematically incline the plate in such a manner that the collodion may run into each corner in succession; when perfectly covered, pour off gently the excess into the bottle at one of the corners nearest to you; with observation and practice dexterity is easily acquired. There are many ways of coating the plate; each person will adopt that which practice teaches him is best. The pneumatic plate-holder is a convenient little instrument to use for holding the plate whilst pouring on the collodion; it may be used for both small and large plates.

Keep the corner of the glass plate in contact with the neck of the bottle whilst pouring off the collodion; otherwise the film will be wavy in places.

4. As soon as the collodion ceases to run, plunge the prepared glass gently, without stopping, into the nitrate of silver bath, which is prepared as follows: Into a 20-oz. stoppered bottle put nitrate of silver, 1-1/4 ozs.; distilled water, 4 ozs.; dissolve. To this solution add iodide of potassium, 4 grs., dissolved in one drachm of distilled water. Mix these two solutions; the precipitate (iodide of silver) thus formed is by shaking entirely dissolved. Add 16 ozs. of distilled water, when the excess of iodide of silver is again thrown down, but in such a finely divided state as to render the saturation of the bath with iodide of silver perfect. Now drop in sufficient of the oxide of silver to turn the turbid yellow solution a dirty brown color; so long as this effect is produced the quantity of oxide of silver, however much in excess, is of no consequence; shake the bottle well for ten minutes or so at intervals; then add alcohol, 30 minims, and filter; to the filtered solution add dilute nitric acid of the strength stated, 5 minims. The bath is now ready for use, and should be quite neutral.

5. Allow the prepared glass to remain in this bath from five to ten minutes, according to the temperature. Move it up and down three or four times whilst in the bath, in order to get rid of the greasy appearance on the surface; drain it, but not too closely. When in the frame, place upon the back a piece of common blotting-paper, to absorb moisture, and the two lower silver wires should also be covered with slips of blotting-paper; after which the sooner it is placed in the camera the better.

6. The time of exposure can only be ascertained by practice--no rules can be laid down; and I am unacquainted with any royal road, but that of experience, leading to constant success in this most important point.

7. The plate having been taken from the camera and placed upon a leveled stand, or held in the hand, develop immediately the latent image with the following solution:

_Iron developing solution._--Protosulphate of iron, 1/4 oz.; glacial acetic acid, 1/4 oz.; spirits of wine, 1/2 oz.; distilled water, 8 ozs.; mix. Pour on of this solution only enough to cover the plate easily, commencing at that edge of the negative which stood uppermost in the camera; move the solution to and fro until it has become intimately mixed with the silver on the plate; then pour off into the developing glass, and _at once_ return it on to the plate. When as much intensity has been obtained as possible with the iron developer, it should be thoroughly removed by washing with water. Any intensity may be obtained afterwards by using either of the following solutions:

8. _Intensifying solution._--Pyrogallic acid, 6 grs.; glacial acetic acid, 1/4 oz.; distilled water, 6 ozs.; mix. A few drops of a 30-gr. solution of nitrate of silver, the quantity to be regulated according to the intensity required, to be added, at the moment of using, to as much of the pyrogallic solution as may be necessary.

_Intensifying solution_ (another form).--1. Pyrogallic acid, 8 grs.; citric acid, 20 grs.; distilled water, 2 ozs. 2. Nitrate of silver, 8 grs.; distilled water, 2 ozs. Mix small quantities of the solutions 1 and 2, in equal portions, the moment before using.

The pyrogallic solution, made with good acetic acid, may be kept for a month or more in a cool place. Nevertheless, if the conditions of light and situation are unfavorable, I should prefer this solution just made. The iron solutions act best when freshly prepared.

It is supposed by some that a prolonged action of the iron developer produces fogginess. This may be the case when impure or improperly prepared collodion is used, but certainly not when the preparation is pure and of the proper quality.

When the image is sufficiently intense, wash freely with common filtered water; then pour on a saturated solution of hyposulphate of soda, which should immediately remove the iodide of silver: wash again well with water; allow as much as the plate will hold to soak in for at least a quarter of an hour, changing the water occasionally, to remove all traces of hyposulphate; lastly, wash the plate with a little distilled water, stand up to dry, and, if required, varnish either with spirit or amber varnish.

The following solution is also very commonly used for fixing the negative:--Cyanide of potassium, 1/4 oz.; water, 12 ozs.

Attention to the following rules and cautions will assist the operator in the production of perfect pictures:--

1. Do not disturb the deposit which will occasionally be found at the bottom of the bottle containing the collodion.

2. Remove all particles of dried film from the neck of the bottle before pouring the collodion on the plate.

3. Never use damp cloths, leathers, or buffs, for giving the final polish to the plate. Negatives with an indistinct and muddy surface are frequently produced from this cause.

4. Let the film set properly before immersion in the nitrate of silver bath: its condition can be ascertained by gently touching the lower part of the coated plate with the end of the finger.

5. Never omit to pass a broad camel-hair brush over the plate just before pouring on the collodion.

6. Bear in mind that, as light is the producing agent, so will it prove a destructive one: not less than four folds of yellow calico should be used to obstruct white light; and in that case the aperture covered should be no larger than is necessary to admit sufficient light for working by. Examine occasionally the yellow calico: when this material is used to exclude white light, it becomes bleached by constant exposure. Do not trust alone to any colored glass; no glass yet made is anti-actinic under all aspects of light and conditions of exposure.

7. When the negative requires intensifying, carefully wash off all traces of the first developing solution before proceeding to intensify. This operation may be performed either before or after the iodide is removed by fixing.

8. Glass baths are preferable to porcelain, ebonite, or gutta-percha baths for solution of nitrate of silver.

9. In using either spirit or amber varnish, before pouring it off, keep the plate horizontal a few seconds. This gives time for soaking in, and prevents the formation of a dull surface arising from too thin a coating.

10. Rub the lenses occasionally with a soft and clean wash-leather, the rapidity of action is much influenced by the brightness of the lenses: their surfaces are constantly affected by moisture in the atmosphere, which condensing, destroys the brilliancy of the image.

11. The white blotting-paper used for some photographic purposes is not suitable for filtering solutions; that only should be employed which is made for this purpose, and is sold under the name of filtering-paper.

12. _Hyposulphate of soda._--A great deal of rubbish is sold under the name of this salt. As a test of its quality, 1-1/2 drachms should entirely dissolve in 1 drachm of water, and this solution should dissolve rather more than 4-1/2 grains of iodide of silver.

13. _Chemicals._--The purity of photographic chemicals cannot be too strongly urged; the cheapest are not always the most economical. The commercial preparations are generally not to be depended upon, as these, though perhaps unadulterated, are, strictly speaking, not chemically pure. It is best to procure them from well-known chemists, who understand the purpose for which they are intended, and make the preparation of these substances peculiarly a branch of their business.

14. Never leave chemical solutions exposed in dishes: when done with, pour them back into glass-stoppered bottles, and decant for use from any deposit, or filter if necessary.

15. In all photographic processes it is absolutely necessary to be chemically clean; and this sometimes is not easy. As a rule, never be satisfied with cleanly appearances only, but take such measures as shall insure the absence of all extraneous matter in preparing the solutions, cleaning the glasses, dishes, etc.

16. All stains on the hands, linen, etc., may be removed by means of cyanogen soap or cyanide of potassium, which should be applied without water at first, then thoroughly washed off. To assist the operation, the hands may be now gently rubbed with a fine piece of pumice-stone, when the stains quickly disappear.

For more perfect and complete directions, the reader is referred to any complete work on photography.

MECHANICS.

There is no subject of such importance as Mechanics, as its principles are founded upon the properties of matter and the laws of motion; and in knowing something of these, the tyro will lay the foundation of all substantial knowledge.

The properties of matter are the following: Solidity (or Impenetrability), Divisibility, Mobility, Elasticity, Brittleness, Malleability, Ductility, and Tenacity.

The laws of motion are as follows:--

1. Every body continues in a state of rest or of uniform rectilineal motion, unless affected by some extraneous force.

2. The change of motion is always proportionate to the impelling force.

3. Action and reaction are always equal and contrary.

EXPERIMENT OF THE LAW OF MOTION.

In shooting at “taw,” if the marble be struck “plump,” as it is called, it moves forward exactly in the same line of direction; but if struck sideways, it will move in an oblique direction, and its course will be in a line situated between the direction of its former motion and that of the force impressed. This is called the resolution of forces.

BALANCING.

The center of gravity in a body is that part about which all the other parts equally balance each other. In balancing a stick upon the finger, or upon the chin, it is necessary only to keep the chin or finger exactly under the point which is called the center of gravity.

THE PRANCING HORSE.

Cut out the figure of a horse, and having fixed a curved iron wire to the under part of its body, place a small ball of lead upon it. Place the hind legs of the horse on the table, and it will rock to and fro. If the ball be removed, the horse would immediately tumble, because unsupported, the center of gravity being in the front of the prop; but upon the ball being replaced, the center of gravity immediately changes as position, and is brought under the prop, and the horse is again in equilibrio.

TO CONSTRUCT A FIGURE, WHICH BEING PLACED UPON A CURVED SURFACE, AND INCLINED IN ANY POSITION, SHALL, WHEN LEFT TO ITSELF, RETURN TO ITS FORMER POSITION.

The feet of the figure rest on a curved pivot, which is sustained by two loaded balls below; for the weight of these balls being much greater than that of the figure, their effect is to bring the center of gravity of the whole beneath the point on which it rests; consequently the equilibrium will resist any slight force to disturb it.

TO MAKE A CARRIAGE RUN IN AN INVERTED POSITION WITHOUT FALLING.

It is pretty well known to most boys, that if a tumbler of water be placed within a broad wooden hoop, the whole may be whirled round without falling, owing to the centrifugal force. On the same principle, if a small carriage be placed on an iron band or rail, it will ascend the curve, become inverted, and descend again, without falling.

TO CAUSE A CYLINDER TO ROLL BY ITS OWN WEIGHT UP-HILL.

Procure a coffee-canister, and loading it with a piece of lead, which may be fixed in with solder, the position of the center of gravity is thus altered. If a cylinder so constructed be placed on an inclined plane, and the loaded part above, it will roll up-hill without assistance.

THE BALANCED STICK.

Procure a piece of wood, about nine inches in length, and about half an inch in thickness, and thrust into its upper end the blades of two pen-knives, on either side one. Place the other end upon the tip of the fore-finger, and it will keep its place without falling.

THE CHINESE MANDARIN.

Construct out of the pith of the elder a little mandarin; then provide a base for it to sit in, like a kettle drum. Into this put some heavy substance, such as half a leaden bullet; fasten the figure to this, and in whatever position it may be placed, it will, when left to itself, immediately return to its upright position.

TO MAKE A SHILLING TURN ON ITS EDGE ON THE POINT OF A NEEDLE.