Thus, Prof. Langley has said that man was thousands of years, and possibly millions, in evolving a cutting edge by rubbing one stone on another; but only a few thousand years to next develop bronze tools, and a still shorter period tools of iron. We cannot say how long the period was from the age of iron tools to the building of the pyramids, but we know that before those stupendous structures arose, the six elementary mechanical powers, the lever, the wheel, the pulley, the inclined plane, the wedge and the screw, were invented. And without those powers, what mechanical tool or machine has since been developed? The age of inventions in the times of the ancients rested mainly upon simple applications of these mechanical powers. The middle ages slumbered, but on the coming of the fifteenth and sixteenth centuries, the inventions of the ancients were revived, new ones added, and their growth and development extended with ever-increasing speed to the present time. The inventions of the nineteenth century, wonderful and innumerable as they are, and marvellous in results produced, are but the fruit of the seed sown in the past, and the blossom of the buds grown upon the stalks of former generations. The early crude stone hatchet has become the keen finished metal implement of to-day, and the latter involves in itself the culmination of a long series of processes for converting the rough ore into the hard and glistening steel. The crooked and pointed stick with which the Egyptian turned the sands of the Nile has slowly grown to be the finished plough that is now driven through the sod by steam. The steam-operated toys of Hero of Alexandria were revived in principle and incorporated in the engines of Papin and the Marquis of Worcester in the seventeenth century; and the better engines of Savery, Newcomen, and more especially of James Watt in the eighteenth century, left the improvements in steam-engines of the nineteenth century--great as they are--inventions only in matter of detail. It has been said that electrical science began with the labours of Dr. Gilbert, published in 1600. These, with the electrical discoveries and inventions of Gray, Franklin, Galvani, and others in the next century, terminating with the invention of his battery by Volta in 1800, constituted the framework on which was built that world of flashing light and earth-circling messages in which we now live. The study of inventions in any one or all eras cannot proceed intelligently unless account is taken not only of their mode of construction, and of their evolution one from another, but of the evolution of distinct arts, their relation, their interdependence in growth, and their mutual progress.