WHEN, in March, 1776, the British evacuated Boston, Washington felt assured that New York, already threatened, would be their objective point, not only on account of its commercial and strategical importance, but because it was the great arsenal of America. He therefore, as soon as practicable, concentrated in and about it his whole disposable force, and pushed forward the defences of the city and of its vicinity, already planned and partly executed by General Lee. Until the arrival of Washington, April 13, 1776, General Putnam commanded at New York, and General Greene, with a considerable body of troops, took charge of the incomplete intrenchments of Brooklyn, extending from the Wallabout (the present Navy Yard) to Gowanus Cove on New York Bay. These were now strengthened by four redoubts armed with twenty pieces of artillery, and by a strong interior keep mounting seven guns. These Brooklyn Heights, from their proximity and command of New York, were considered the key of the defence of this valuable city. Fort George, with several redoubts and batteries, guarded the southern end of Manhattan Island, while the fortified hills overlooking Kingsbridge protected its northern extremity. On Red and Paulus Hooks, and at various points along the shores of the East and Hudson rivers, were erected earthworks, and a strong redoubt was built upon Governor's Island. Between the latter and the "Battery", hulks were sunk to obstruct the main channel. Notwithstanding all these defences, Manhattan Island, as events proved, was assailable at many points. To defend these works, scattered over more than twenty miles, Washington had an army of only 17,225 men, of whom 6,711 were sick, on furlough, or detached, leaving but 10,514 present for duty. Most of these were militia, badly clothed, imperfectly armed, without discipline or military experience, and their artillery was old and of various patterns and calibres. There had been dispatched from England a powerful fleet under Lord Howe, convoying a large body of troops to reinforce those already in America. The army of General William Howe (brother of the Admiral) on Staten Island in August (including some 8,600 German hirelings) numbered, as stated by General Clinton, 31,625 rank and file, of whom 24,464 were well-appointed, disciplined soldiers, fit for duty and equal to any in Europe. The struggle for the Hudson, by the coöperation of the army of Canada with Howe, was now about to begin; but Washington was at his wits' end to foresee the particular point upon which the blow would fall. Hence he was obliged to retain the greater part of his troops in New York to defend the city, holding them ready, however, to support any point in the vicinity whether assailed by the enemy's large fleet or by their powerful army. [Illustration: THE MORTIER HOUSE, RICHMOND HILL. (_Washington's Headquarters._)