THE

HE element selenium, when in crystalline form, possesses the peculiar property of being electro-sensitive to light. It is a good or bad conductor of electricity according to the intensity of the light that falls upon it, and its response to variations of illumination is virtually instan

taneous.

This interesting property has been utilized in a wide variety of applications, ranging from the transmission of pictures over a telegraph line to the automatic detecting of comets; but by far the most marvelous application is that of the phonopticon, which was exhibited in San Francisco this summer at the annual convention of the American Association of Workers for the Blind. It is an apparatus that will actually read a book or a newspaper, uttering a characteristic combination of musical sounds for every letter it scans.

The principle of operation is not difficult to understand. A row of, say, three tiny selenium crystals is employed, each crystal forming part of a telephone circuit leading to a triple telephone-receiver. In each circuit there is an interrupter that breaks up the current into pulsations, or waves, of sufficient frequency to produce a musical note in the receiver. The frequency differs in the three circuits, so that each produces its characteristic pitch. Although the conductivity of selenium is increased by intensifying its illumination, the electrical connections in this apparatus

are so chosen that while the crystals are illuminated no sounds are heard in the telephone, but when the crystals are darkened, there is an instant audible response.

The apparatus is placed on the printed. matter that is to be read, with the row of crystals disposed at right angles to the line of type. The paper directly under the crystals is illuminated by a beam of light. This is reflected from the unprinted part of the paper with sufficient intensity to keep the telephone quiet, but when the crystals are moved over the black printing, the light is diminished, and the crystals lose their conductivity, causing the telephone to respond with a set of sounds. which vary with the shape of the letter. Suppose the apparatus was being moved over the letter V, the upper crystal would encounter the letter first, then the middle one would respond, next the lower one would come into action for an instant, followed by a second response of the middle crystal and a final response of the upper crystal. A set of notes would be sounded somewhat after this fashion: mi, re, do, re, mi. The sound combinations with such letters as S and O are more complicated, but they are distinguishable. When we read with the natural eye we do not spell out the words letter by letter, but recognize them by their appearance as a whole. In the same way with the mechanical eye entire words can be recognized after a little practice.

An apparatus akin to this and known as

the optophone was invented by Fournier d'Albe and exhibited before the Royal Society of London last year, but it would read only specially prepared copy printed on one face of the sheet, because the light was passed through the paper. The phonopticon was invented by Dr. F. C. Brown of the State University of Iowa, and owes its sensitiveness to the use of individual crystals of selenium in place of the mass of crystalline material heretofore used. Dr. Brown spent four years trying to obtain selenium crystals large enough to be handled individually.

Of course the phonopticon is yet in the laboratory stages, but it offers every prospect of practical success, and its possibilities are untold. It is quite conceivable that the apparatus may be elaborated to such an extent that a blind man may see (by ear) where he is going. His world. may never be bathed in sunshine, but he may learn to admire the beauties of nature as translated from light into music.

METAL-PLATING WITH A "PISTOL"

A FEW years ago some children in Zurich were practising with a small rifle when their father strolled up to examine their target and see how well they were doing. He noticed that a number of the little bullets had struck so closely that they overlapped, and were mashed together, like dough, into a solid mass of lead. It was not an extraordinary phenomenon, but it was an inventor who witnessed it, and what he saw was a new method of metalplating.

The man was M. U. Schoop, a Swiss engineer, and this is how he conceived the idea of plating objects by peppering them with a storm of microscopic bullets. Instead of using cold bullets, however, he took molten metal, and sprayed it upon the object with a stream of heated compressed air. The air picked up the metal in a finely divided state, just as an atomizer picks up perfumery, and projected it as a fine mist. Strange to say, this metallic mist was so cool that it could be deposited upon the most delicate fabrics without burning them. Even the hand could be

held in the spray close to the nozle without being scorched, and the metal could be laid on in as thin or as thick a coating as desired, just as paint is applied with an air-brush. So successful was this first metal spraying-machine that Mr. Schoop was encouraged to build a small hand machine, shaped somewhat like a pistol, in which powdered, instead of molten, metal was used. The metal powder was fused by a gas torch, while the compressed air that fed the torch picked up the molten particles and shot them forth in the

form of a spray. Another development

was to use a metal ribbon or wire fed to the torch by a small air-driven motor.

The latest modification of the platingpistol uses electricity in place of gas. Two wires of the metal that is to be applied take the place of the carbon pencils of a small arc lamp. The arc melts the wire, and a blast of compressed air blows out a continuous stream of atomized metal. Within the pistol is the mechanism commonly used in arc lamps to feed the pencils to the arc as fast as they are consumed.

It is not easy to explain the peculiar action of the spray. The metal is certainly hot as it leaves the pistol, but it is so finely pulverized that the microscopic particles must lose their heat almost instantly, like the sparks that fly from a grindstone. Expanding air absorbs heat, and no doubt the air of the jet, being suddenly relieved of its compression, extracts the heat from the metal particles, so that within a few inches of the nozle they are solidified, and, on striking the object, are mashed together just like the bullets on the target.

In the first apparatus a very high airpressure was used, which shot forth the particles with a speed of over a mile per second, which is about twice the speed of a rifle-bullet and nearly five times the velocity of sound. Now a much lower air-pressure is used, and the "bullets" leave the pistol with the moderate speed. of seventeen hundred feet per second, or nearly twelve hundred miles per hour. Of course their velocity is greatly reduced before they reach their target. So tiny are the particles that it is possible to form

a film one thousandth of an inch in thickness; that is, about one third of the thickness of the paper this is printed on. Of Of course the longer the spray is applied the thicker the plating will grow, and there is no limit to the depth of metal that may be laid on.

It is only two years since the first patents were secured on this method of plating, and already it has found a wonderful variety of useful applications. Lead, tin, zinc, aluminium, copper, nickel, iron, and even silver and gold have been used with success, and coatings of alloys such as brass, bronze, German silver, and the like, which cannot be applied electrolytically, may be sprayed on with the pistol. "Pickling," or chemical cleaning, is not a prerequisite of the Schoop process, hence many objects that cannot be electroplated may be spray-plated. It is just as easy to apply metal to the finest silks as to a block of wood or iron, and highly combustible. materials, such as celluloid, tissue-paper, and even matches, have been subjected to a hail of microscopic bullets without being ignited. The delicacy of the process may be seen in its use for making molds for dental plates, and in taking casts of finger-tips for the identification of criminals, in place of the common finger-print system. On the other hand, it has been used to coat telegraph-posts at the ground-line in order to protect them from decay. It is reported that, owing to the scarcity of copper, German arms factories are making cartridge-shells of paper coated with a thin layer of brass, applied by the new process.

THE LEWIS AIR-COOLED GUN

FOR a great peace-loving country, far removed from militaristic influences, American contributions to the machinery of war have been really remarkable. This is particularly true of the machine-gun. It was an American, Dr. Richard J. Gatling, who invented the first rapid-fire gun-a gun with a cluster of barrels that would discharge a stream of bullets at the turning of a crank handle. That was in 1861, several years before the French mitrail

leuse made its appearance. Then, twenty years later, came the single-barreled machine-gun, in which each cartridge was loaded and fired by the recoil of the previous cartridge, at the rate of ten shots per second. This was invented by Hiram Maxim, an American, who later became a British subject. Now we have a third notable American contribution in the aircooled gun of I. N. Lewis, a retired captain of the United States army, which is so light that it can be handled by a single man, and may even be fired from the shoulder like a regular service rifle, although it takes a strong man to do this, for it weighs twenty-six and one half pounds.

The automatic machine-gun and the automobile engine have many points of close resemblance. The engine cylinder has its counterpart in the gun-barrel, the piston in the bullet. In each an explosive is introduced and fired, and part of the energy of the explosion is used to clear the cylinder and introduce and fire a new charge, this process being repeated hundreds of times per minute. So frequent are the explosions that the intensely hot fire of burning gases is almost continuous, and, like the engine cylinder, the gun-barrel must be provided with a cooling system to keep its temperature within safe limits.

In the Gatling gun no special system of cooling was required, because, having a number of barrels, there was an appreciable interval of time between discharges in each barrel, during which the heat could be dissipated. In the Maxim gun the barrel is surrounded by a water jacket that holds about two gallons of water. It takes only six hundred rounds to bring this water to a boil, and about a pint and a half is evaporated for every thousand shots. The Colt machine-gun, another American design by the way, depends upon air cooling, or radiation of the heat directly into the surrounding atmosphere. In order to provide a sufficient radiating surface, the barrel is made with a very thick wall, giving it a large outside diameter, and hence an extensive superficial

area.

The Hotchkiss gun has radiating fins on the barrel something like the fins of the motor-cycle or air-cooled automobile engine. The Lewis gun still further approximates the air-cooled automobile engine by employing a counterpart of the cooling-fan to draw a current of cool air over the radiating fins. The latter are of aluminium, and run longitudinally along the barrel. They are surrounded by a casing of sheet metal, which is open at the rear, while its forward end tapers to a smaller diameter and extends beyond the muzzle of the gun. As the burning gases that speed the bullet on its way burst out of the muzzle they spread through a flaring cup on the end of the barrel, and, sweeping through the contracted portion of the casing, draw in their wake a current of air that absorbs and carries off the heat in the fins and barrel.

clears the muzzle the pressure of the gases in the gun-barrel is also felt in the cylinder. This impulse of pressure throws back a piston in the cylinder. The movement of the piston opens the breach and ejects the spent cartridge, while a fresh one takes its place. It also winds up a spring, which reacts, and throws the piston back to its original position, while at the same time it closes the breach upon the next cartridge and fires it. By adjusting the port in the barrel, the pressure in the cylinder can be regulated so as to retard or accelerate the speed of firing. The rate can be varied between 350 and 750 shots per minute. One of the advantages of giving the gases additional work to do, particularly that of drawing air through the casing, is that the recoil is cut down very materially, so that it is possible to fire the gun at arm's-length.

Unlike other machine-guns, the Lewis fires cartridges from a pan-shaped magazine instead of from a cartridge-belt. The magazine holds about fifty cartridges, arranged in two layers and set radially. A single sharp pull on the trigger will fire only one shot; but if the trigger is held back, the gun will keep on firing until the fifty cartridges have been discharged, which, at the maximum speed, will take only four seconds. In two seconds the empty magazine can be removed, and replaced by a filled one. The time that elapses between shots is, at the minimum, only eight hundredths of a second, and in this interval the magazine must be turned to position, the cartridge dropped out of it and moved into the barrel, the breach closed, the hammer cocked, the trigger pulled, the cartridge fired, the empty shell extracted and ejected from the gun, and the parts restored to position for the next cartridge.

As in the air-cooling system, it is the burning gases that actuate the mechanism. A very small part of their energy is utilized for this purpose. Close under the barrel is a cylinder that communicates with the bore of the gun near the muzzle. During the brief interval after the bullet passes this opening and before it fully

While these mining-machines have not been introduced with any humanitarian object, they are bettering the lot of the miner by eliminating dynamite and the frightful disasters that attend the use of explosives where combustible gases and dust are likely to accumulate.

There is another very efficient substitute for the dynamite cartridge, which may abolish blasting even in hard-rock mines. It is a hydraulic cartridge, or an apparatus that works on the principle of the hydraulic jack. Unlike dynamite, which consists of a lot of stored and highly concentrated energy that is let fly to do what destruction it may, the hydraulic cartridge is absolutely inert and devoid of potential energy when placed in the blasthole. Only after it is in place is the en

ergy applied to it. This it gradually accumulates until it acquires enough to burst open the rock without wasting a lot of energy in pulverizing it. The apparatus is under the direct control of the miner all the time. There is nothing haphazard about its operation.

The cartridge consists of a strong steel cylinder, made in various sizes. Disposed at right angles to the length of the cylinder are a number of pistons, or rams, that may be forced out laterally by pumping water into the cylinder. The cartridge is introduced into the blast-hole with the rams retracted. Then a quick-action pump is operated to move the rams out so that they come into contact with the rock. After this, by means of a screw lever, a powerful pressure is exerted upon the water, which forces out the rams until the rock gives way under the strain.

It is hard to realize that a man can exert a pressure at all comparable with that of dynamite. If we hark back to our school-days, we may recall that a pressure on one point of a confined liquid will be felt throughout the liquid equally in all directions. For instance, if a bottle with an inside area of one hundred square inches and a mouth opening of one square inch were completely filled with water, a closely fitting stopper bearing upon the water with a pressure of only one pound would produce a total pressure of one hundred pounds on the wall of the bottle. So in the hydraulic cartridge, by operating with a very powerful screw leverage upon a small area of water, a heavy total pressure may be exerted upon the rams. Pressure of three, four, and even five tons to the square inch is not unusual.

Fortunately, rock is so inelastic that only a slight movement of the rams is required to produce a fracture. The stress is exerted in the plane of movement of the ram, and so the direction of fracture may be controlled to a large extent; whereas with the dynamite cartridge it is usually im possible to tell beforehand in what direction the rock is going to give way. The pump and the screw piston with which the hydraulic pressure is produced are con

nected to the cartridge by a pipe of suitable length to permit the operator to stand beyond danger from falling fragments of the shattered rock. Nevertheless, he can work very near the cartridge. It is a decided advantage to be able to "shoot a hole" without calling off all the men from the heading and herding them behind shelters, or waiting for the noon whistle to clear them out of the workings.

CELLULOSE CASINGS FOR SAUSAGES

Ir is natural to look upon imitations with distrust, for it is seldom that they offer any advantages to the consumer other than cheapness. Recently there has been introduced into the markets a substitute for an old and venerable article of food, and the imitation is so perfect that no one would ever question its genuineness.

From time immemorial the gullet and intestines of the ox and the intestines of the sheep and the hog have been used as containers for sausage meat. Had they not been established as a food staple by generations of use, we should shrink from them in disgust, as we do from the Chinaman's bird's-nest soup.

Now sausage casings are being made out of wood-pulp. The material is chemically treated, so that it shows not the slightest trace of a fibrous structure. It is reduced to a jelly-like mass of cellulose, and then formed into transparent tubes that are no thicker than the usual casings and have a smooth, glossy surface which furnishes no lodgment for mold.

Of course cellulose has no food value, but the casings can be masticated, which is more than can be said of the envelops of large sausages such as the Bologna. As a matter of fact, in only the smaller sizes is the entire sausage eaten.

For sanitary as well as esthetic reasons the cellulose envelops are a distinct improvement. To be sure, the purity of casings produced by the large packinghouses cannot be questioned, but the product of the small butcher shop may not be above suspicion, whereas the very process of their manufacture renders the cellulose casing aseptic.