T stands motionless on an ordinary railroad track, an ordinary gondola car, such as is used for the transportation of coal, only it is about half the customary size. Not a man is within a thousand feet of it. Suddenly it starts and moves down a grade dipping at an angle of twenty degrees. Now most cars would rush down such a declivity at a speed which would become cyclonic as the foot was neared. This little car actually slows its pace and gently crawls to the bottom, where, after pausing a moment, it starts again and continues along about its business. That involves stopping, while it is loaded, starting and climbing that incline again, this time with a load of ten tons.

The spectacle of an inanimate structure of wood and iron doing all this apparently of its own volition may be seen any day at a big stone quarry on the Drainage Canal south of Chicago. It seems none the less uncanny when the observer is told that a man in a tower on the edge of the pit controls the movements of the car. The operation in fact is one of the multitudinous performances of electricity when harnessed by modern engineers.

At this quarry, an area of thirty acres has been excavated to a depth of fifty feet. Around the sides of it runs a railroad which continues up the edge to level ground, where it describes a loop that carries it through the crusher plant and past the loading platform. Thus the track is

like a figure 8 with the upper or smallest circle on the surface. The track is divided electrically into sections, each independent of the rest and each fed with current over a third rail in the middle. In your wonderment at the intelligent movements of the car you haven't noticed that. Neither have you remarked that the car is equipped with a motor and in addition a variety of simple but remarkable devices without which it couldn't perform as it does. The control of everything centers in a bank of switches in the

NO MOTORMAN IS ON THIS LITTLE ELECTRICALLY PROPELLED CAR. WHICH IS TRAVELING UP A STIFF GRADE WITH A TEN-TON LOAD.

When the car nears the brink of the big hole in the earth the motor is working away merrily, without a sign to indicate that it is anything but a motor. Just as it crosses the brink the pole changer inclines, striking an angle iron frame which shifts a lever, changing the connections on the car, so that the motor is converted into a generator. In its changed character it goes to work immediately after the fashion of electrical machinery, a resistance coil comes into play and the motion of the car is so retarded that it slips down the grade as though held back by a wire rope slowly paid out. When the bottom is reached the generator transforms itself into a motor again, current is thrown into the next track section and the car moves on.

Its business is to receive a load of broken stone from one of the huge mechanical shovels you see at work. It stops at exactly the right place to receive its load. How is that managed? By means of an automatic brake. A current ninety volts is used for braking, while 250 volts is needed to actuate the motors.

The car has still more duties to perform and proceeds about it in a semihuman manner. When the loading is finished a signal is given the tower man -a signal because the distance is too great to see clearly. He pulls a switch. The car loaded with broken stone runs around the pit works up the grade to the surface as easily as it rode down and moves into the building containing the crusher. It registers itself on entering

THE TOWER FROM WHICH THE CARS ARE CONTROLLED.

tions miles away. Sometimes it is necessary to build a sub-station on the ground. In this particular plant all the heavy machinery in the building is run by electricity. The huge shovels scooping up stone are worked by current. So are the numerous drills you see hammering shot holes in the rock. The dynamite shots are all fired by the man in the tower. Even the great whistle that screams out that a shot is about to be exploded and that men in the danger zone must seek places of safety, is brought into action by a switch under his hand.

A

LTHOUGH it is a matter of little moment to most of us whether the tides of the ocean are high or low, to navigators, engineers who are calculating marine improvements, and hydrographic engineers who wish to lay plans for future surveys, it is a matter of intense interest and they eagerly consult the predictions of Uncle Sam's tide prophet. For a number of years the tide predictions have been the combined work of human and mechanical computers, but there has recently been completed and put in operation a machine in the United States Coast and Geodetic Survey, at Washington, D. C., which outclasses all competitors, for at one operation it gives the depth of water at any instant on any date from one to two years in the future, and longer if necessary. It is the inven

tion of Mr. E. G. Fisher, who has been striving for fourteen years to bring the machine to its present state of perfection. It looks very much like a huge printing press; its great skeleton frame of brass, steel and iron occupies a space eleven feet long, six feet high, and two feet wide, and from the dial indicators in the front to the opposite end it is a labyrinth of wheels, gears, pulleys, and chains. There are nearly three hundred gear wheels and pulleys in the machine, which are arranged in two main sections, one representing the time and the other the height of the tide. Two chains, each permanently fastened at one end, run through each section and their free ends are attached to indicating devices.

Tide predictions are based on certain conditions that influence the rise and fall of the oceans. At every port in the world

there is an instrument which indicates their fluctuations by a curved line on a sheet of paper, and from a long series of such observed curves computers have evolved the average of each component at each place. The formulae upon which Mr. Fisher's machine is based includes thirty-seven of these components. There is a set of gears to represent the sun's influence, another to denote that of the moon, others which act for the planetary movements, and still other sets which indicate local conditions.

and lowest tides, another pen is tracing a curved line which shows the gradual rise and fall.

Mother Nature sometimes upsets all man's predictions for the far future, an easterly storm at, say, the port of New York, may force the water from the ocean into the harbor and cause a tide of abnormal height, which cannot be foreseen more than three or four days in advance. To insure the usefulness of tidal information under such conditions Mr. Fisher constructed an instrument called a tide indicator, which is now in use in the principal ports of the United States. It resembles the upper half of the face of a clock which measures twenty-five feet in radius. The height of the tide is indicated on the dial by a pointer connected to a float and the movement of the tide is shown by means of two black movable triangles, which when drawn together form an arrowhead. As the tide rises the arrowhead points upward. If it is at a stand they will point toward each other and if it is falling they will point downward. This instrument can be seen for a distance of five miles.

The operator when beginning a set of predictions turns a crank which causes the wheels to rise or descend, thus lengthening or shortening the free end of the chain. This variation appears on the dials at the front of the machine, one pointer showing the height of water in feet and tenths, the other the day, hour, and minute of the occurrence. An electrical device stops the machine whenever high or low water is indicated, which thus tells to the operator what he should record in his table of predictions. While the figures are being recorded the machine is doing a still more detailed work. A strip of paper six inches wide and 380 feet long is moving automatically across the face of the machine, and while one pen is marking the hours and exact times of highest

IGHT can be dangerous. There are times when, either through its either through its intensity or by reason of the heat that is generated in its making,

the blessing may become a menace of a terrible sort. One needs to think only of the cinematograph to recall plenty of accidents and disasters resulting from the handling of light in close proximity to inflammable films. Eliminating the element of peril, there are processes in which light, by radiating heat, defeats the purposes to which it is put. Certain delicate operations of surgery are in this category. So is the study of living microscopic organisms under the lens and the light. And these only suggest the multitude of cases in which a light without heat would be of invaluable help to science.

M. F. Dussaud, of Paris, has produced what he calls "a box of cold light," and appears to have solved the difficult problem in a very simple and effective way.

He has already applied his device to the magic lantern, the cinematograph, the microscope, surgery, and the making of luminous signs and color photography.

The lantern is not cumbersome in shape or size. It has two projectors, which enable the operator to get graduated light effects that are impossible in the old style instruments. Its "cold" light is obtained simply by the use of an intermittent current of electricity. And so simple are its requirements that its small bulb, with metallic filament, may be supplied with current either from the ordinary house-supply or from a portable battery.

Most important is the apparatus which has been devised to accomplish the interruption of the current in such periods as to give the "cold" result. The intermittent current causes pulsations in the thread of the bulb, so that the light is broken up into a multitude of swift successive flashes, as it were, instead of