Part 6

This friend’s name is “Hypo,” and in a few minutes he has carried away all the little bromides that are still sleeping, so that the trundle bed with the now awakened and transformed silver bromides will, after washing and drying, be called a negative, and ready to print your pictures from.

If we take this negative, as it is called, and hold it up to the light, we will see that everything is reversed, not only from right to left, but also that whatever is white or light in color is dark in the negative, and that what would correspond to the darker parts of our picture are the lightest in the negative, and it is from these facts that we give it the name negative.

Now, to get our picture as it should be, we must place this negative in contact with a sheet of coated paper that is also sensitive to light.

So we place the negative and the sheet of sensitive paper in what is called a printing frame, with the negative uppermost, so that the light may shine through the negative, and impress the image upon the sheet of sensitive paper. Now, it stands to reason that if the lightest parts of our picture are the darkest in the negative that less light can pass through such portions of the negative in a given time, so that with the proper exposure to light the image upon the sheet of sensitive paper will be a correct picture of whatever the lens saw.

TWENTY-FIVE MILES A MINUTE

AN EXCLUSIVE STORY, ILLUSTRATED WITH A SERIES OF REMARKABLE PHOTOGRAPHS TAKEN WITH THE FASTEST CAMERA IN THE WORLD

BY CLEVELAND MOFFETT

~HOW SHOOTING SHELLS ARE PHOTOGRAPHED~

One of the most progressive branches of our military service is the Department of Coast Defenses, which, under the far-seeing guidance of General E. M. Weaver, holds our shores and harbors in a state of alert preparedness against foreign aggression. At Hampton Roads sits the Coast Artillery Board, composed of officers and consulting engineers to whom are referred all problems relating to coast artillery, and who have the responsibility of testing all new instruments proposed for artillery use. The purpose of this article is to describe one among several notable achievements of the Hampton Roads Coast Artillery School, this particular work having been done by Captain F. J. Behr of the Coast Artillery Corps, who, after years of effort, has recently developed a system that makes it possible to take pictures of the swiftest moving bodies, the great steel projectiles of our biggest guns--to seize them with the camera’s eye as they hurl through the air at enormous velocities or at the very moment of their emergence from the gun muzzles, and to preserve these images, never seen before, for military study and comparison. Captain Behr was ably assisted in this work by Engineer J. A. Wilson.

The big gun, equipped with the fastest camera shutter in the world, about to be fired and the shell photographed.

For years a young officer of the Coast Artillery has been trying to devise a camera so incredibly swift that it will record every stage of this lightning flight from the gun-barrel to the target. At last he has succeeded. His photographs--some of them taken one hundred thousandth of a second apart--have revealed remarkable and unsuspected facts to the military world. The story of his invention had never before been told.]

Reckoning in Millionths of a Second.

Some of the increments and decrements of time involved in the series of photographs herewith published (several of them for the first time) are as small as one ten-thousandth part of a second. And Captain Behr has devised a method of taking photographs of projectiles as they arrive at a steel target and penetrate the target, inch by inch, that involves increments or decrements of time as small as the one hundred-thousandth part of a second. To the uninitiated it seems incredible that such infinitesimal divisions of time can be used in practical calculations; but every trained physicist knows that in wireless work scientists of to-day speak casually of experiments that take account of two-tenths or one-tenth of a millionth part of a second!

In this photograph--the first of a remarkable series showing five stages of a moving projectile--the half-ton projectile seems to be standing still, but really it is traveling at the rate of 900 miles an hour. The gunners here work in concrete pits 34 feet high. Underneath the mounts are the powder magazines. Each pit has four mortars usually served by an entire Coast Artillery Company. The projectiles are the same as those used in the twelve-inch guns, but less powder is required because mortar projectiles are hurled high in the air, not straight at a vessel, and deliver their destructive blows downward from a great height.]

This second photograph shows the projectile almost entirely out of the mortar. Its sharp nose may be seen above the “gas-ring” forming at its upper end. These “gas-rings,” or “smoke-rings,” come without warning, and only occasionally, perhaps once in eight or ten shots. They rise swiftly to the height of fifty or a hundred feet, growing larger and larger, and giving forth a weird, shrieking sound like a second projectile. Some insist that these “smoke-rings” are as hard as steel, owing to the enormous compression of their composing gases, and the story is told of a bird caught in the path of one of them and torn to pieces.]

What happened to the projectile after it leaves the gun, or after the discharge of the gun, and before the projectile has had time to issue from the gun-barrel? What is the action at the muzzle of gases generated? What shape do these gases assume as they leave the gun? What causes the much-discussed “gas-rings” that sometimes form when a mortar is fired, and oftener do not form? What phenomena attend the arrival of the projectile at a solid steel target? Is the steel actually fused by the heat of impact? Is it vaporized? Or what? These are some of the questions that Captain Behr set himself to solve, or to help in solving, as he worked out his methods of rapid photography. His aims were strictly military, but his results make fascinating appeal to the general imagination. Fancy doing anything in the one hundred-thousandth part of a second!

In the third photograph the smoke-cone is almost perfect and gives the famous “powder-puff” effect. It still hides the projectile, although the latter is traveling at a velocity that would take it from New York to Chicago in one hour. At night the “gas-rings” present a startling and fascinating appearance, burning with a reddish orange glow, and whirling with a complicated double motion, strange opalescent balls, like rings of Saturn. A study of these photographs--the first record ever made of the “gas-rings”--has led some experts to the conclusion that the cause of the rings is defective ramming of the projectile.]

The fourth photograph shows the projectile emerging from the smoke-cone about thirty feet above the muzzle of the mortar. The men who fire these mortars from the mortar-pits never see the distant target or vessel they are firing at, but point their mortars according to directions transmitted to them (usually by telephone) from observers at distant stations. And so great a degree of precision has been attained that, on certain practice occasions at Hampton Roads, a record of nine hits out of ten shots has been scored on a moving target five miles out in the ocean. This picture shows the smoke-cone as first seen by the human eye.]

Captain Behr’s general idea was to utilize some phenomena connected with the discharge to actuate, by electrical connections, a mechanism that would work a rapid shutter in a properly placed camera. The phenomenon of concussion was tried first--the smash of air against a little swinging door; but this was much too slow. The projectile was hundreds of yards away before the camera had registered its picture. And that chance was gone!

In the fifth photograph the projectile is seen entirely clear of the smoke-cone and well started on its long flight. Climbing into the sky at this steep angle, it will reach a height of from three to six miles before it begins to descend. There are harbors on our coasts guarded by so many guns and mortars that if these were fired simultaneously they could hurl against a given small area a converging rain of projectiles aggregating more than fifty tons in their combined mass. A minute later they could hurl another fifty tons against the same small area; and so on as long as the ammunition lasted.]

In the next trial, several months later, Captain Behr arranged to have the electrical connections made or broken by the movement of the gun-carriage itself in recoiling; but the result was unsatisfactory. Nor was he more fortunate at the succeeding target practice, when, having placed the apparatus farther forward on the parapet, he had the camera demolished by the force of the concussion and several blades of the rapid shutter broken. He was satisfied, now, that his effort to actuate the camera mechanism from the gun-carriage would never give the requisite precision in results, and he saw that he must work with a device functioning more reliably.

In the months that followed before the next target practice, the Captain did some experimenting, and finally determined making the projectile itself displace a length of piano-wire fixed across the muzzle of the gun, and thus actuate the electrical system and operate the shutter. In this way he eliminated troublesome variables of recoil, elasticity of the carriage, etc., leaving to determine only the time element of the electrical system to function. This result was admirable, and, after taking several similar pictures, the captain found that he could now operate with great precision--that is, he could get the same phase of the discharge with almost identical shapes of gas-cone and smoke-cloud, and he could get these every time.

In the fall of 1912 Captain Behr succeeded in obtaining a series of extremely rapid photographs showing a twelve-inch mortar battery in action. In taking these pictures the camera was placed on an elevation about ten feet above the concrete floor and about sixty feet back of the mortars. The electrical device for working the shutter was actuated by the mortar itself in its recoil. These pictures were taken in about one five-thousandth of a second--which is the more remarkable as the last two were taken in the shade after 4.30 A.M. The first three were taken about noon, in the sunshine, as the shadows show.

So great was the precision of the electrical device as to render possible the photographic recording of these mortar projectiles, moving at great velocities, in almost any desired position after the discharge, say two feet away from the muzzle, or six feet away, or twenty feet away, or right at the muzzle, as shown in the first mortar picture, where the great projectile has been caught in its flight half way out of the mortar.

Pictures Never Seen By the Human Eye.

~A CAMERA THAT IS FASTER THAN THE EYE~

It is interesting to note that of these five mortar pictures, representing five phases of the firing, only the last two are ever seen by the human eye. The far swifter camera, acting in about one five-thousandth of a second, has caught all these phases as reproduced here; but, to the ordinary observer standing by, the first visible impression after firing is that of the smoke-cone as developed in Number Four. The strange “powder-puff” effect shown in Number Three is never seen; nor the earlier effects in Numbers One and Two. Nor is any sound heard by an observer or by the gun crew until the third or fourth phase has been reached. This is a matter of simple calculation.

Sound travels through the air very slowly as compared with light, and in Numbers One, Two, and Three, although the crashing explosion has taken place and the projectile is already started on its long journey, the men (even the lanyard man, who is nearest), have heard nothing, since the sound-waves have not yet had time to reach their ears. Nor has the mortar itself had time to recoil, as it does presently, down into the well in the floor of the pit.