We can perhaps learn the cause of it better by a comparison than by direct study of bibliographical history. In Voltaire's time thermometers were so great a rarity that the owner of one of them was considered to be a savant. Time and social progress have so completely altered this state of things that thermometers are now made in factories, are owned by all classes, and applied to the commonest uses. The thermometers hanging on our walls no longer indicate familiarity with science, but merely that a new tool has been added to household appliances. So in book-making. The art which once served chiefly to record discoveries in knowledge, conduct controversies in polemics, philosophy, and politics, and for other grave and important purposes now adds to these a multitude of common uses. A library may contain scores and even hundreds of volumes, and yet have nothing but those books which have served in the education and amusement of the children in an ordinary family. Or it may be the result of a chance aggregation of "railway literature," bought to relieve the tediousness of travel. Or it may consist, as is sometimes the case, of the small and precious collections in frontier log huts, of the gratuitous contributions of the patent medicine vender, the plough-maker, and the lightning-rod man, mingled with the dear-bought subscription books of the wandering peddler! Books are so common that the possession of them is no longer an indication of the intellectual tendency of their possessors. With libraries open to the public the case is different. Their condition affords one standard by which the character and tastes of the people may be measured.