These pumps are also built with steam cylinders both of the plain slide valve and Corliss designs. THE ELECTRIC PUMP [Illustration: COMPASS NEEDLE—See PAGE 266.] ELECTRICITY AND ELECTRICAL MACHINERY. Each kind of power requires its own special machinery so constructed and adapted as to utilize it; hence, to be serviceable to mankind, electricity demands machinery suited to its nature; what that is, will be indicated in the following few paragraphs. Electricity is a name derived from the Greek word _electron_—amber. It was discovered more than 2,000 years ago that amber when rubbed with a Fox’s tail possessed the curious property of attracting light bodies. It was discovered afterwards that this property could be produced in a dry steam jet by friction, and in A. D. 1600 or thereabouts, that glass, sealing-wax, etc., were also affected by rubbing, producing electricity. Whatever electricity is, it is impossible to say, but for the present it is convenient to consider it as a kind of invisible something which pervades all bodies. While the nature and source of electricity are a mystery, and a constant challenge to the inquirer, many things about it have become known—thus, it is positively assured that electricity never manifests itself except when there is some mechanical disturbance in ordinary matter, and every exhibition of electricity in any of its multitudinous ways may always be traced back to a mass of matter. NOTE.—The great forces of the world are invisible and impalpable; we cannot grasp or handle them; and though they are real enough, they have the appearance of being very unreal. Electricity and gravity are as subtle as they are mighty; they elude the eye and hand of the most skillful philosopher. In view of this, it is well for the average man not to try to fathom, too deeply, the science of either. To take the machines and appliances as they are “on the market,” and to acquire the skill to operate them, is the longest step toward the reason for doing it, and why the desired results follow. Electricity, it is also conceded, is without weight, and, while electricity is, without doubt, one and the same, it is for convenience sometimes classified according to its motion, as— 1. _Static electricity_, or electricity _at rest_. 2. _Current electricity_, or electricity _in motion_. 3. _Magnetism_, or electricity _in rotation_. 4. _Electricity in vibration._ Other useful divisions are into— 1. _Frictional_ and 2. _Dynamical_, And into— 1. _Static_, as the opposite of 2. _Dynamic electricity._ There are still other definitions or divisions which are in every-day use, such as “vitreous” electricity, “atmospheric” electricity, “resinous” electricity, etc.