Great inventions are never, and great discoveries are seldom, thework of any one mind. Every great invention is really either an aggregation of minor inventions, or the final step of a progression. It is not a creation, buta growth—as truly so as is that of the trees in the forest. Hence, the same invention is frequently brought out in several countries, and by several individuals, simultaneously. Frequently an important invention is made before the world is ready to receive it, and the unhappy inventor is taught, by his failure, that it is as unfortunate to be in advance of his age as to be behind it. Inventions only become successful when they are not only needed, but when mankind is so far advanced in intelligence as to appreciate and to express the necessity for them, and to at once make use of them.