On 2 Mar 1776, the Continental Army began an artillery barrage against Boston, firing from Cobble Hill and Lechmere’s Point in Cambridge and Lamb’s dam in Roxbury. This was the opening of a spring offensive designed to drive the British military away from the town.
Remember how excited the Americans were back in November after capturing a British ordnance ship? The biggest prize was a thirteen-inch brass mortar that Gen. Israel Putnam and Quartermaster-General Thomas Mifflin christened “the Congress.” That was deployed for this bombardment, and was probably one of the first guns to be fired.
In his history of Middlesex County, Samuel Adams Drake wrote:
It was related by Colonel [William?] Burbeck that the battery containing the “Congress” mortar was placed under the command of Colonel David Mason. With this mortar Mason was ordered to set fire to Boston. His first shell was aimed at the Old South, and passed just above the steeple.Gen. William Heath of Roxbury wrote that the mortars “were not properly bedded, as the ground was hard frozen.” The Americans probably still lacked experienced artillerists, and were paying for it.
The next shell was aimed more accurately at the roof, which it would doubtless have entered had not the mortar burst, grievously wounding the colonel and killing a number of his men. . . .
Through the inexperience of those who served them, four other mortars were burst during the bombardment which preceded the occupation of Dorchester Heights.
I suspect Drake relied on Richard P. Frothingham’s History of the Siege of Boston in counting five mortars burst over all. As of 3 Mar 1776, Heath and Dr. James Thacher had counted only three, so Frothingham may have counted two twice. Still, those deadly explosions must have been demoralizing for the Americans.
That bombardment was only the first part of the Continental commanders’ plan, however. The artillery fire was meant to keep the British busy while Americans fortified the heights on Dorchester point. The image above, from the Dorchester Atheneum, shows how that town’s peninsula overlooked the Boston peninsula to its northwest (so small it’s not even labeled on this map) as well as a narrow point in the harbor.
At the Battle of Bunker Hill, the provincials had started their redoubt in the middle of the night before the battle. The provincials improvised more protection for themselves along a rail fence, and had superior numbers available, but they ran out of gunpowder and couldn’t stop the British from taking not only the redoubt but the whole Charlestown peninsula.
Gen. George Washington and his commanders were determined to make the Dorchester fortifications strong enough to withstand a British counterattack. That required preparing parts of the works in advance, to be assembled on the heights, and a multi-day construction effort. Hence the need to distract the British with cannonballs and shells.