by the same means to the ground. Buffers of all kinds have been devised to effect the stoppage of the carrier without injury thereto under the different degrees of force with which it is moved upon its way, to prevent rebounding, and to enable the carrier to be discharged with facility at the end of its route.

Among the early mechanical means of transporting the carriage was an endless cable moved continuously by an engine, and this adoption of cable principle in store service was co-eval with its adoption for running street cars. Also the system of switching the cars from the main line to a branch, and in different parts of a city, at the same time that all lines are receiving their motive power from the main line, corresponds to the manner of conveying cash to all parts of a building at the same time from many points.

To the great department store or monstrous building wherein, as we have said, the whole business of a town may be transacted, the assemblage and conjoint use of elevators and conveyors seem to be actually necessary.

A very useful and important line of inventions consists in means for forming connections between rotary shafts and their pulleys and mechanisms to be operated thereby, by which such mechanism can be started or stopped at once, or their motion reversed or retarded; or by which an actuating shaft may be automatically stopped. These means are known as clutches.

They are designed often to afford a yielding connection between the shaft and a machine which shall prevent excessive strain and wear upon starting of the shaft. They are also often provided with a spring connection, which, in the rotation of the shaft

in either direction, will operate to relieve the strain upon the shaft, or shafts, and its driving motor. Safety clutches are numerous, by which the machine is quickly and automatically stopped by the action of electro-magnets should a workman or other obstruction be caught in the machinery.

Electric auxiliary mechanism has also been devised to start or stop the main machine slowly, and thus prevent injury to small or delicate parts of complicated machines, like printing presses for instance. Clutches are arranged sometimes in the form of weights, resembling the action of the weights in steam governors, whereby centrifugal action is relied upon for swinging the weights outward to effect a clutching and coupling of the shaft, or other mechanism, so that two lines of shafting are coupled, or the machine started, or speeded, at a certain time during the operation. In order to avoid the great mischief arising sometimes from undue strain upon and the breaking of a shaft, a weak coupling composed of a link is sometimes employed between the shaft and the driven machine, whereby, should the force become suddenly too great, the link of weaker metal is broken, and the connection between the shaft thereby destroyed and the machine stopped.

To this class of inventions, as well as to many others, the phrase, "labour-saving", is applied as a descriptive term, and as it is a correct one in most instances, since they save the labour of many human hands, they are regarded by many as detrimental to a great extent, as they result in throwing out of employment a large number of persons.

This derangement does sometimes occur, but the curtailment of the number of labourers is but temporary after all.

The increased production of materials, resulting from cheaper and better processes, and from the reduced cost of handling them, necessitates the employment of a larger number of persons to take care of, in many ways, the greater output caused by the increased demand; the new machinery demands the labour of additional numbers in its manufacture; the increase in the size and heights of buildings involves new modes of construction and a greater number of artisans in their erection; new forms of industry springing from every practical invention which produces a new product or results in a new mode of operation, complicates the systems of labour, and creates a demand for a large number of employers and employees in new fields. Hence, it is only necessary to resort to comparative statistics (too extensive to cite here) to show that the number of unemployed people in proportion to the populations, is less in the present age than in any previous one. In this sense, therefore, inventions should be classed as labour-increasing devices.

CHAPTER XI.

HYDRAULICS.

THE science of Hydraulics appears to be as old as the thirst of man.

When prehistoric men had only stone implements, with which to do their work, they built aqueducts, reservoirs and deep wells which rival in extent many great similar works that are the boast of their modern descendants. Modern inventors have also produced with a flourish nice instrumentalities for raising water, agencies which are covered with the moss of untold centuries in China.

It was more than an ancient observation that came down to Pliny's time for record, that water would rise to a level with its source. The observation, however, was put into practical use in his time and long before without a knowledge of its philosophical

cause.

Nothing in Egyptian sculpture portraying the arts in vogue around the cradle of the human race is older than the long lever rocking upon a cleft stick, one arm of the lever carrying a bracket and the other arm used to raise a bucket from a well. Forty centuries and more have not rendered this device obsolete.

Among other machines of the Egyptians, the Carthagenians, the Greeks, and the Romans, for raising water was the tympanum, a drum-shape wheel divided into radial partitions, chambers, or pockets, which were open to a short depth on the periphery

of the wheel, and inclined toward the axis, and which was driven by animal or manual power. These pockets scooped up the water from the stream or pond in which the wheel was located as the wheel revolved, and directed it toward the axis of the wheel, where it ran out into troughs, pipes, or gutters. The Noria, a chain of pots, and the screw of Archimedes were other forms of ancient pumps. The bucket pumps with some modifications are known in modern times as scoop wheels, and have been used extensively in the drainage of lands, especially by the Dutch, who at first drove them by windmills and later by steam.

The division of water-wheels into overshot, undershot and breast wheels is not a modern system.

In the Pneumatics of Hero, which compilation of inventions appeared in 225 B. C., seventy-nine illustrations are given and described of simple machines, between sixty and seventy of which are hydraulic devices. Among these, are siphon pumps, the force pump of Ctesibius, a "fire-pump," having two cylinders, and two pistons, valves, and levers. We have in a previous chapter referred to Hero's steam engine. The fact that a vacuum may be created in a pump into which water will rise by atmospheric pressure appears to have been availed of but not explained or understood.

The employment of the rope, pulley and windlass to raise water was known to Hero and his countrymen as well as by the Chinese before them. The chain pump and other pumps of simple form have only been improved since Hero's day in matters of detail. The screw of Archimedes has been extended in application as a carrier of water, and converted into a conveyor of many other materials.