Thenceforward, Washington, by the exercise of imagination, constructiveness and sheet force of will, fought a continual fight against forces that were superior in material and training, but inferior in mentality. Finally, in August, 1781, the crisis came. The British were occupying New York, and Washington was in front of it, threatening to attack it, but knowing that he could not do so with success. About August 14 he received a letter written in July by Admiral Comte de Grasse, then in the West Indies, saying that he would start with his fleet and a force of troops for Chesapeake Bay on August 13. Washington knew that the British General Cornwallis was entrenched at Yorktown, near the mouth of the Chesapeake, with a force considerably inferior to his own. He instantly proceeded to embody in action an idea that he had already conceived--that of leaving the vicinity of New York secretly, and marching with the utmost possible despatch to Yorktown, and calling on de Grasse to assist him to capture Yorktown, and if possible Cornwallis. No invention ever succeeded better. Its influence on history was to precipitate the collapse of the entire British program of hostilities, and cause the establishment of the United States. The balloon was invented about 1783. Mr. Cavendish had found that hydrogen was about seven times lighter than air, and Dr. Black had forthwith delivered a lecture in which he pointed out that a thin light vessel inflated with hydrogen should be able to rise and float in the air. He conceived the idea of the balloon, but made no invention. The Italian philosopher, Cavallo, about 1782, inflated soap-bubbles with hydrogen gas, but went no further. The subject of making balloons filled with hydrogen was widely discussed; but the first balloon really to rise was the hot-air balloon invented by Joseph and Stephen Montgolfier. This balloon made a successful ascent on June 5, 1783, carrying the two brothers, flew about ten minutes, and alighted safe, after a trip of about a mile and a half. This was followed on August 27 by a flight of a balloon filled with hydrogen gas, the design of which was made by the physicist Charles, and the cost of which was met by a popular subscription. The flight was followed shortly by many others. The first employment of balloons in practical work was in making observations of the enemy by the French army in 1794. An important invention for utilizing mechanical power in place of hand-power was the power-loom invented in 1785 by Edmund Cartwright. This was an invention of the most clean-cut kind, originating in the conception by the Rev. Dr. Cartwright of the possibility of doing much more weaving by mechanical power than by hand, then constructing the machine to accomplish it, and then accomplishing it. An interesting fact in the early development of looms for weaving was the determined and angry opposition of weavers to each improvement in succession.