undergone a remarkable revolution in the past century. Though fewer hands are needed for working the presses in proportion to the amount of printing done, the increase of employment to compositors, bookbinders etc. has ' been very great. The manufacture of printing-machinery, as well as the manufacture of paper, has itself become a great industry, and no one can safely predict what progress will be made in this field during the 20th century. See The American Dictionary of Printing and Bookmaking.

Prism, an optical instrument composed of two plane, refracting surfaces. The angle between these two planes is called the refracting angle of the prism. Since in any kind of matter, except ether, lights of different color travel with different speeds, it follows that lights of different color will each have a different direction impressed upon them on passing through a prism. A prism thus produces " dispersion " in a beam of white light and is used, therefore, in the spectroscope to separate the various colors "which compose the incident beam. Any transparent material whose refractive index is desired is generally cut in the form of a prism. ; for on measuring the angle of deviation for any ray, call it D, and the refracting angle of the prism, which we may call A, we have the following relation :

sin (Af£)

Refractive index=-------------------

sin (-£.) See Light, Luxfer Prism and Spectroscope.

Pris'oners of War are persons captured from the enemy during military or naval operations. In ancient times the tr atment of prisoners of war was very severe, it being no uncommon thing to put the population of a conquered city or state to the sword or to reduce them to slavery. Although the practice of putting prisoners to death became less frequent as the principles of Christianity became more widely diffused, they and their families were commonly reduced to slavery as late as the 13th century. By degrees the more humane custom of exchanging and paroling prisoners was adopted, although in all wars there still is more or less of suffering among prisoners.

Pris'ons, strange to say, are a development of civilization, and also of personal liberty. The abolition of slavery and the decay of the feudal system made it necessary in all countries to take the punishment of crime out of private hands and turn the offender over to the power of the state. In the exercise of this powei governments have punished their subjects by death, direct torture, transportation, exposure in the pillory etc. All these, except death, have been abandoned by most of the civilized nations; and in all of them death is inflicted only for the most

serious offenses; and, as a consequence, imprisonment for a greater or lesser period is resorted to as a means of protection to society. How crudely, how barbarously, this power has often been exercised has in part been revealed to the world If the whole story could be told, how dark would be the picture! It is a relief to turn from the contemplation of the miseries and horrors of prison-life to the efforts that are being made by prison-reformers throughout the world to reduce prison-discipline to a humane system and make it a means of reformation to the prisoner as well as a means of protection to the public. The prisons of the world are yet very far from perfection; but in every civilized nation there are earnest men and women studying all the questions connected with prison-reform, and th results already are seen in a more intelligent and Christian system of discipline. In the United States, especially, our jails and penitentiaries have been relieved of many degrading features, and in a number of the states reform-schools have been established for juvenile offenders. In 1877 New" York established a reformatory at Elmira for the reception of all prisoners under 30, in which, while strict order and discipline are main-tai'ed, the inmates are carefully trained and educated and encouraged to earn a parole and even a discharge by good conduct and thorough reformation. In Ohio, in addition to a reform-shool for girls at Delaware and a reform-farm for boys at Lancaster, an intermediate penitentiary has been established at Mansfield, which is conducted after the manner of Elmira Reformatory. Similar steps have been taken in other, states. See The State of Prisons in the Civilized World by Wines.

Pri'vateer, a ship owned by a private individual or by a number of individuals, which, under letters of marque (a. v.), makes wari upon the shipping-commerce of a hostile power. Privateering was abolished among European nations, except Spain, by the treaty of Paris in 1856; but the United States never signed this agreement. During the War of 1812 and to some extent during the Revolutionary war British commerce suffered severely at the hands of American privateers, there being over 200 of these afloat during 1812-14. In the Civil War the Confederate privateer, Alabama, captured a considerable number of United States merchant-vessels, for which England afterwards paid damages for allowing a privateer to depart from one of her ports. By act of congress, passed in 1864, the entire proceeds of a prize taken by a privateer go to the captors, unless otherwise provided and specified in the ship's commission, the sum awarded being divided equally between the ship's company and her owners. See Alabama Claim.