Page:EB1911 - Volume 22.djvu/238

Rh performed several Irish plays, and published three novels and his Impressions of America (1836). He had married when twenty and left a widow and seven children, the oldest of whom, Sir William Tyrone Power, K.C.B. (b. 1819), became Commissary-general of the British army and was knighted in 1865.

POWER OF ATTORNEY, or, is an authority under hand and seal empowering the person named therein to do some act on behalf of the principal, which otherwise could only be done by the principal himself. It is either general or special. A general power of attorney authorizes the agent to act for his principal in all matters, or in matters of a particular nature only, or in respect of a particular business. A special act of attorney authorizes the agent to represent his principal only in some particular specified act. It expires with death of the principal, and is revocable at his will, even by a verbal notice, unless it has been given for a valuable consideration. Moreover, the terms of the power are construed literally, and give such authority only as they confer expressly or by necessary implication. The Conveyancing Act of 1881 provides protection for any person making any payment or doing any act in good faith, in pursuance of a power of attorney, if before the time of the payment or act the donor of the power had died or become lunatic, of unsound mind, or bankrupt, or had revoked the power. The law relating to powers of attorney is a branch of the law of agency. (See ; .)

 POWERS, HIRAM (1805–1873), American sculptor, the son of a farmer, was born at Woodstock, Vermont, on the 29th of June 1805. In 1819 his father removed to Ohio, about six miles from Cincinnati, where the son attended school for about a year, staying meanwhile with his brother, a lawyer in Cincinnati. After leaving school he found employment in superintending a reading-room in Connexion with the chief hotel of the town, but, being, in his own words, “forced at last to leave that place as his clothes and shoes were fast leaving him,” he became a clerk in a general store. His second employer in this line of business having invested his capital in a clock and organ factory, Powers set himself to master the construction of the instruments, displaying an aptitude which in a short time enabled him to become the first mechanic in the factory. In 1826 he began to frequent the studio of Mr Eckstein, and at once conceived a strong passion for the art of sculpture. His proficiency in modelling secured him the situation of general assistant and artist of the Western Museum, kept by a Frenchman named Dorfeuille, where his ingenious representation of the infernal regions to illustrate the more striking scenes in the poem of Dante met with extraordinary success. After studying thoroughly the art of modelling and casting, at the end of 1834 he went to Washington, where his remarkable gifts soon awakened general attention. In 1837 he settled in Florence, where he remained till his death. While he found it profitable to devote the greater part of his time to busts, his best efforts were bestowed on ideal work. In 1839 his statue of “Eve” excited the warm admiration of Thorwaldsen, and in 1843 he produced his celebrated “ Greek Slave, ” which at once gave him a place among the leading sculptors of his time. Among the best known of his other ideal statues are the “Fisher Boy,” “Il Penseroso,” “Proserpine,” “California,” “America” (modelled for the Crystal Palace, Sydenham), and the “Last of his Tribe.” He died on the 27th of June 1873.

POWER TRANSMISSION. The appliances connected with installations for the utilization of natural sources of energy may be classified into three groups:—

1. Prime movers, by means of which the natural form of energy is transformed into mechanical energy. To this group belong all such appliances as water turbines, steam turbines, steam engines and boilers, gas producers, gas engines, oil engines, &c.

2. Machinery of any kind which is driven by energy made available by the prime mover. To this group belong all machine tools, textile machinery, pumping machinery, cranes-in fact every kind of machine which requires any considerable quantity of energy to drive it.

3. The appliances by means of which the energy made available by the prime mover is transmitted to the machine designed to utilise it. The term power is used to denote the rate at which energy is transmitted. The unit of power in common use is the horse power, and one horse power means a rate of transmission of 550 foot-pounds per second.

In many cases the prime mover is combined with the machine in such a way that the transmitting mechanism is not distinctly differentiated from either the prime mover or the machine, as in the case of the locomotive engine. In other cases the energy made available by the prime mover is distributed to a number of separate machines at a distance from the prime mover, as in the case of an engineer's workshop. In this case the transmitting mechanism by means of which the energy is distributed to the several machines has a distinct individuality. In other cases prime movers are located in places where the natural source of energy is abundant, namely, near waterfalls, or in the neighbourhood of coal-fields, and the energy made available is transmitted in bulk to factories, &c., at relatively great distances. In this case the method and mechanism of distribution become of paramount importance, since the distance between the prime mover and the places where the energy is to be utilized by machines is only limited by the efficiency of the mechanism of distribution)

Prime movers are considered in the articles ; various special articles. The methods and mechanisms of distribution or transmission alone form the subjects of the present article, and the different methods in general use readily fall into four divisions:—  
 * , and and machines in

§ 1. Methods.—The mechanical transmission of power is effected in general by means of belts or ropes, by shafts or by Wheel gearing and chains. Each individual method may be used separately or in combination. The problems involved in the design and arrangement of the mechanisms for the mechanical distribution of power are conveniently approached by the consideration of the way in which the mechanical energy made available by an engine is distributed to the several machines in the factory. By a belt on the fly-wheel of the prime mover the power is transmitted to the line shaft, and pulleys suitably placed along the line shaft by means of other belts transmit power, first, to small counter shafts carrying fast and loose pulleys and striking gear for starting or stopping each engine at will, and then to the driving pulleys of the several machines. (See also .)

§ 2. Quantitative Estimation of the Power Transmitted.—In dealing with the matter quantitatively the engine crank-shaft may be taken as the starting point of the transmission, and the first motion-shaft of the machine as the end of the transmission so far as that particular machine is concerned.

Let T be the mean torque or turning effort which the engine exerts continuously on the crank shaft when it is making N revolutions per second. It is more convenient to express the revolutions per second in terms of the angular velocity, that is, in radians per second. The relation between these quantities is ＝2N. Then the, rate at which work is done by the engine crank shaft is T foot-pounds per second, equivalent to T/550 horse power. This is now distributed to the several machines in varying proportions. Assuming for the sake of simplicity that the whole of the power is absorbed by one machine, let T1 be the torque on the first motion shaft of the machine, and let wl be its angular velocity, then the rate at which the machine is absorbing energy is T11 foot-pounds per second. A certain quantity of energy is absorbed by the transmitting mechanism itself for the purpose of overcoming frictional and other resistances, otherwise the rate of absorption of energy by the machine would exactly equal the rate at which it was produced by the prime mover assuming steady conditions of working. Actually therefore T11 would be less than T so that

