This is the result of a knowledge of Practical Astronomy. "Place an astronomer," says Mr. Newcomb, "on board a ship; blindfold him; carry him by any route to any ocean on the globe, whether under the tropics or in one of the frigid zones; land him on the wildest rock that can be found; remove his bandage, and give him a chronometer regulated to Greenwich or Washington time, a transit instrument with the proper appliances, and the necessary books and tables, and in a single clear night he can tell his position within a hundred yards by observations of the stars. This, from a utilitarian point of view, is one of the most important operations of Practical Astronomy."[2] The Marine Chronometer was the outcome of the crying want of the sixteenth century for an instrument that should assist the navigator to find his longitude on the pathless ocean. Spain was then the principal naval power; she was the most potent monarchy in Europe, and held half America under her sway. Philip III. offered 100,000 crowns for any discovery by means of which the longitude might be determined by a better method than by the log, which was found very defective. Holland next became a great naval power, and followed the example of Spain in offering 30,000 florins for a similar discovery. But though some efforts were made, nothing practical was done, principally through the defective state of astronomical instruments. England succeeded Spain and Holland as a naval power; and when Charles II. established the Greenwich Observatory, it was made a special point that Flamsteed, the Astronomer-Royal, should direct his best energies to the perfecting of a method for finding the longitude by astronomical observations. But though Flamsteed, together with Halley and Newton, made some progress, they were prevented from obtaining ultimate success by the want of efficient chronometers and the defective nature of astronomical instruments. Nothing was done until the reign of Queen Anne, when a petition was presented to the Legislature on the 25th of May, 1714, by "several captains of Her Majesty's ships, merchants in London, and commanders of merchantmen, in behalf of themselves, and of all others concerned in the navigation of Great Britain," setting forth the importance of the accurate discovery of the longitude, and the inconvenience and danger to which ships were subjected from the want of some suitable method of discovering it. The petition was referred to a committee, which took evidence on the subject. It appears that Sir Isaac Newton, with his extraordinary sagacity, hit the mark in his report. "One is," he said, "by a watch to keep time exactly; but, by reason of the motion of a ship, and the variation of heat and cold, wet and dry, and the difference of gravity in different latitudes, such a watch hath not yet been made."