There may be nothing but water and sky within his view; he may be in the midst of the ocean, or gradually nearing the land; the curvature of the globe baffles the search of his telescope; but if he have a correct chronometer, and can make an astronomical observation, he may readily ascertain his longitude, and know his approximate position--how far he is from home, as well as from his intended destination.

He is even enabled, at some special place, to send down his grappling-irons into the sea, and pick up an electrical cable for examination and repair.
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.