It seemed to be the principle employment of both armies to look at each other with spyglasses

From 1776 by David McCulough. Page 27.

Not the least of Washington’s problems was that he had command of a siege, yet within his entire army there was not one trained engineer to design and oversee the building of defenses. Still, he ordered larger and stronger defenses built, and the work went forward. “Thousands are at work every day,” wrote the Reverend William Emerson of Concord after touring the lines. “ ’Tis surprising the work that has been done…. ’Tis incredible.” It had been the Reverend Emerson who declared the morning of April 19, as British regiments advanced on Concord, “Let us stand our ground. If we die, let us die here!”

With telescopes from Prospect Hill and other vantage points, the army kept constant watch on the regulars in Boston, just as the regulars kept watch on the army. (“It seemed to be the principle employment of both armies to look at each other with spyglasses,” wrote the eminent Loyalist Peter Oliver, former chief justice of the province.)