In the general plan it was concerted that, should the enemy detach a large force to dislodge our men from Dorchester Heights, as had been done in the affair of Bunker's Hill, an attack upon the opposite side of the town should forthwith be made by General Putnam. For this purpose he was to have four thousand picked men in readiness, in two divisions, under Generals Sullivan and Greene. At a concerted signal from Roxbury, they were to embark in boats near the mouth of Charles River, cross under cover of the fire of three floating batteries, land in two places in Boston, secure its strong posts, force the gates and works at the Neck, and let in the Roxbury troops.
The evening of Monday, the 4th of March, was fixed upon for the occupation of Dorchester Heights. The ground was frozen too hard to be easily intrenched; fascines, therefore, and gabions, and bundles of screwed hay, were collected during the two preceding nights, with which to form breastworks and redoubts. During these two busy nights the enemy's batteries were cannonaded and bombarded from opposite points, to occupy their attention, and prevent their noticing these preparations. They replied with spirit, and the incessant roar of artillery thus kept up, covered completely the rumbling of wagons and ordnance.
As soon as the firing commenced, the detachment under General Thomas set out on its cautious and secret march from the lines of Roxbury and Dorchester. Everything was conducted as regularly and quietly as possible. A covering party of eight hundred men preceded the carts with the intrenching tools; then came General Thomas with the working party, twelve hundred strong, followed by a train of three hundred wagons, laden with fascines, gabions, and hay screwed into bundles of seven or eight hundred weight. A great number of such bundles were ranged in a line along Dorchester Neck on the side next the enemy, to protect the troops, while passing, from being raked by the fire of the enemy. Fortunately, although the moon, as Washington writes, was shining in its full lustre, the flash and roar of cannonry from opposite points, and the bursting of bombshells high in the air, so engaged and diverted the attention of the enemy, that the detachment reached the heights about eight o'clock without being heard or perceived. The covering party then divided; one half proceeded to the point nearest Boston, the other to the one nearest to Castle Williams. The working party commenced to fortify, under the directions of Gridley, the veteran engineer, who had planned the works on Bunker's Hill. It was severe labor, for the earth was frozen eighteen inches deep; but the men worked with more than their usual spirit; for the eye of the commander-in-chief was upon them. Though not called there by his duties, Washington could not be absent from this eventful operation.
The labors of the night were carried on with activity and address. When a relief party arrived at four o'clock in the morning, two forts were in sufficient forwardness to furnish protection against small-arms and grape-shot; and such use was made of the fascines and bundles of screwed hay, that, at dawn, a formidable-looking fortress frowned along the height. Howe gazed at the mushroom fortress with astonishment, as it loomed indistinctly, but grandly, through a morning fog. "The rebels," exclaimed he, "have done more work in one night than my whole army would have done in one month."
Washington had watched with intense anxiety the effect of the revelation at daybreak. An American, who was on Dorchester Heights, gives a picture of the scene. A tremendous cannonade was commenced from the forts in Boston and the shipping in the harbor. "Cannon shot," writes he, "are continually rolling and rebounding over the hill, and it is astonishing to observe how little our soldiers are terrified by them. The royal troops are perceived to be in motion, as if embarking to pass the harbor and land on Dorchester shore, to attack our works. The hills and elevations in this vicinity are covered with spectators, to witness deeds of horror in the expected conflict."
General Thomas was reinforced with two thousand men. Old Putnam stood ready to make a descent upon the north side of the town, with his four thousand picked men, as soon as the heights on the south should be assailed. As Washington rode about the heights, he reminded the troops that it was the 5th of March, the anniversary of the Boston massacre, and called on them to revenge the slaughter of their brethren. They answered him with shouts.
Howe, in the meantime, was perplexed between his pride and the hazards of his position. In his letters to the ministry, he had scouted the idea of "being in danger from the rebels." He had "hoped they would attack him." Apparently, they were about to fulfil his hopes, and with formidable advantages of position. He must dislodge them from Dorchester Heights, or evacuate Boston. The latter was an alternative too mortifying to be readily adopted. He resolved on an attack, but it was to be a night one.
In the evening the British began to move. Lord Percy was to lead the attack. Twenty-five hundred men were embarked in transports, which were to convey them to the rendezvous at Castle William. A violent storm set in from the east. The transports could not reach their place of destination. The men-of-war could not cover and support them. A furious surf beat on the shore where the boats would have to land. The attack was consequently postponed until the following day. That day was equally unpropitious. The storm continued, with torrents of rain. The attack was again postponed. In the meantime, the Americans went on strengthening their works; by the time the storm subsided, General Howe deemed them too strong to be easily carried; the attempt, therefore, was relinquished altogether.
What was to be done? The shells thrown from the heights into the town proved that it was no longer tenable. The fleet was equally exposed. Admiral Shuldham, the successor to Graves, assured Howe that if the Americans maintained possession of the heights, his ships could not remain in the harbor. It was determined, therefore, in a council of war, to evacuate the place as soon as possible. But now came on a humiliating perplexity. The troops, in embarking, would be exposed to a destructive fire. How was this to be prevented? General Howe's pride would not suffer him to make capitulations; he endeavored to work on the fears of the Bostonians, by hinting that if his troops were molested while embarking, he might be obliged to cover their retreat by setting fire to the town.
The hint had its effect. Several of the principal inhabitants communicated with him through the medium of General Robertson. The result of the negotiation was that a paper was concocted and signed by several of the "select men" of Boston, stating the fears they had entertained of the destruction of the place, but that those fears had been quieted by General Howe's declaration that it should remain uninjured, provided his troops were unmolested while embarking; the select men, therefore, begged "some assurance that so dreadful a calamity might not be brought on, by any measures from without."