We must drive them from their post, or desert the place.

From 1776 by David McCulough. Page 92.

Little damage was done by the exchange. It was nearly all noise, just as Washington wished, and the night would have been reckoned a complete success were it not that three of the big mortars burst, due apparently to the inexperience of Henry Knox and his artillerymen.

Sunday night the firing resumed and again the British responded with full crescendo. On the third and crucial night of Monday, March 4, the roar of the guns from both sides became more furious by far.

British captain Charles Stuart described sheets of fire filling the sky. But as he also recorded, “The inhabitants were in a horrid situation, particularly the women, who were several times drove from their houses by shot, and crying for protection.” Watching from the American lines, Lieutenant Samuel Webb wrote, “Our shells raked the houses and the cries of the poor women and children frequently reached our ears.”

At the first crash of the guns, General Thomas and 2,000 men started across the Dorchester causeway, moving rapidly and silently, shielded from view by the long barrier of hay bales. An advance guard of 800 men, a “covering party” made up largely of riflemen, went first, to fan out along the Dorchester shores in case the British made any attempt to investigate during the night. The main work party of 1,200 men followed immediately after, and then came hundreds of carts and heavy wagons loaded with chandeliers, fascines, hay bales, barrels, and most important of all, the guns from Ticonderoga.

“The whole procession moved on in solemn silence, and with perfect order and regularity, while the continued roar of cannon serves to engage the attention and divert the enemy,” wrote Dr. Thacher, who, crossing the causeway with the troops, noted with gratitude the “vast number of large bundles of screwed hay arranged in a line next [to] the enemy…to which we should have been greatly exposed while passing.”

Progress up the steep, smooth slopes was extremely difficult, yet numbers of the ox teams and wagons made three and four trips.

The night was unseasonably mild—indeed, perfectly beautiful with a full moon—ideal conditions for the work, as if the hand of the Almighty were directing things, which the Reverend William Gordon, like many others, felt certain it was. “A finer [night] for working could not have been taken out of the whole 365,” he wrote. “It was hazy below [the Heights] so that our people could not be seen, though it was a bright moonlight night above on the hills.”

At Cambridge, on the moonlit Common, Generals Greene and Sullivan paraded with 4,000 troops in front of the college buildings, ready to move to the river and the shallow draft boats in the event of a signal from the Roxbury church steeple.

Recounting the night’s events later, General Thomas would say it was as early as ten o’clock when the fortifications on the Heights were sufficiently ready to defend against small arms and grapeshot. It was also about ten when a British lieutenant colonel, Sir John Campbell, reported to Brigadier General Francis Smith that the “rebels were at work on Dorchester Heights.” It was news that called for immediate action. But Smith, a stout, slow-moving, slow-thinking veteran of thirty years’ service, chose to ignore it; and from that point on, the work proceeded unnoticed by any other British officers or troops on guard, or by any of the Loyalists in Boston who served as willing eyes and ears for the British.

On the Heights the men toiled steadily with picks and shovels, breaking the frozen ground for earth and stone to fill the chandeliers and barrels. At three in the morning a relief force of 3,000 men moved in, and an additional five regiments of riflemen took up positions near the shore. By the first faint light before dawn, everything was ready, with at least 20 cannon in place.

It was an utterly phenomenal achievement. General Heath was hardly exaggerating when he wrote, “Perhaps there never was so much work done in so short a space of time.”

At daybreak, the British commanders looking up at the Heights could scarcely believe their eyes. The hoped-for, all-important surprise was total. General Howe was said to have exclaimed, “My God, these fellows have done more work in one night than I could make my army do in three months.”

The British engineering officer, Archibald Robertson, calculated that to have carried everything into place as the rebels had—“a most astonishing night’s work”—must have required at least 15,000 to 20,000 men. Howe, in his official account, would be more conservative and put the number at 14,000.

Later that spring, one of the London papers would carry portions of a letter attributed to an unnamed “officer of distinction at Boston":

5th March. This is, I believe, likely to prove as important a day to the British empire as any in our annals. We underwent last night a very severe cannonade, which damaged a number of houses, and killed some men. This morning at day break we discovered two redoubts on the hills of Dorchester Point, and two smaller works on their flanks. They were all raised during the night, with an expedition equal to that of the genie belonging to Aladdin’s wonderful lamp. From these hills they command the whole town, so that we must drive them from their post, or desert the place.