After his marriage, Thompson took his place as one of the wealthiest inhabitants of the district in which he resided, and mixed in the best society which the colony afforded. It was not long before he made the acquaintance of his Excellency John Wentworth, the governor of the colony, who, anxious, no doubt, in the critical circumstances in which the American dependencies of Great Britain were then placed, to attach to the party which sided with the mother country as many influential colonists as he was able, lost no time in endeavoring to gain over so promising a man as Thompson. A vacancy having occurred in a regiment of the New Hampshire militia, Governor Wentworth gave the commission, which was that of major, to his new friend: an act of attention which, while it seems to have been gratifying to Thompson, did not fail to procure him much ill-will from the officers already in the service, over whose heads he had been promoted. From this period Thompson began to be unpopular in his native province. He was represented as a friend of Great Britain, and an enemy to the interests of the colonies; and this charge was the more readily believed, on account of the marked kindness with which he continued to be treated by the governor, and the indifference which he exhibited to those political questions which were agitating all around him. The truth seems to be, that not only was Thompson, as a man in comfortable circumstances, and fond of the consideration and opportunities of enjoyment which they afforded him, averse to any disturbance, such as a war between the colonies and the mother country would cause, but that his constitution and temperament, his liking for calm intellectual pursuits, disqualified him from taking part in political agitation. Many men who have distinguished themselves in literature and science have, as a matter of principle, kept themselves aloof from the controversies and political dissensions of their time, alleging that, however important such questions might be, it was not in discussing them that their powers could be employed to most advantage. In the case of Thompson, however, who as yet had not begun to lay claim to the character of a man devoted to scientific pursuits, his countrymen thought, not altogether unreasonably, that they had grounds of complaint. What employment was he engaged in, that he ought to be exempted from the duty of a citizen—that of taking an interest in public affairs? So, probably, the most candid and considerate of the American patriots reasoned; and as for the great mass of the populace, they condemned him in the usual summary manner in which the public judges. Not a name was more detested in Massachusetts than that of Benjamin Thompson. He was denounced as a sycophant of the British—a traitor to the interests of the colonies—an enemy of liberty. To such a length did the public hatred of him proceed, that at length, in the month of November 1774, the mob of Concord had resolved to inflict on him the punishment which several other unpopular persons had already experienced—that of being tarred and feathered in the open streets. Receiving intelligence of the design of the mob before it could be carried into execution, Thompson had no alternative but to withdraw from Concord to some other part of the provinces where political excitement did not run so high. Accordingly, he quitted his wife and an infant daughter, who had been born in the previous year, and took refuge first in his native town of Woburn, from which he afterwards removed to Charleston. From Charleston, after a few months residence, he went to Boston, which was then garrisoned by a British army commanded by General Gage.
Thompson was well received by General Gage and the officers of the British army; and his intercourse with them, while it probably gave him a stronger bias towards the side of the mother country than he had yet exhibited, did not contribute to remove the bad opinion his countrymen had formed of his patriotism. Having returned in the spring of 1775, to his native town of Woburn, where he was joined by his wife and daughter, he again ran the risk of being tarred and feathered. The mob surrounded the house where he resided early one morning, armed with guns and sticks, and but for the interference of his old friend Loammi Baldwin, who arrived at the spot in time to use his influence with the crowd, serious consequences might have ensued.
The commencement of open hostilities between the colonists and the British troops in May 1775, made Thompson’s position still more critical. As a major in the militia of the province, he would probably have acted on the side of the patriots, obeying the orders of the Provincial Congress, which had superseded the old government; but the odium attached to his name was such, that his very zeal on the patriotic side would have been misrepresented. In order, therefore, to clear himself of all suspicion, and that he might thenceforth live on good terms with his countrymen, he demanded a trial before the Committee of Correspondence established at Woburn by authority of the new power. The trial was granted: he was put under arrest; and an advertisement was inserted in the newspapers for all who had charges to prefer against his patriotism to come forward. Besides the general allegation of his being a Tory, and a friend and correspondent of Governor Wentworth and General Gage, the only charge made against him on his trial was, that he had been instrumental in sending back to their colors two British deserters, having procured their pardon from General Gage during his residence in Boston. This, which ought properly to have been regarded as a mere act of mercy, was construed in a less favorable manner by Thompson’s judges; and although, on the conclusion of his trial, the court declared that he had done nothing which could legally be considered as a crime, he was set at liberty without the satisfaction of a full and formal acquittal. Against this treatment he protested in the strongest manner, insisting that he should either be punished as guilty, or declared innocent; but his protests were unheeded.
With a view, apparently, to convince his countrymen of his patriotism by actual service, or possibly because he could enjoy more quiet in the army than the ill-will of his fellow-citizens would allow him in his own house, Thompson, as soon as his trial was over, joined a detachment of the troops of Congress stationed at Chelsea. ‘In the hopes of obtaining a commission,’ says his biographer, ‘he paid great attention to tactics, and assisted at the drills of the yet undisciplined forces. He also took up the study of fortification, which he pursued with his usual ardor. Towards the close, however, of the summer of 1775, his position had become irksome, and even dangerous. Suspicions, which it seemed impossible to allay, shut against him all access to military rank in the continental army. He now could not go from place to place within the lines of the army, without being pointed at as the famous Tory Thompson; and though military discipline sheltered him from actual violence, he was exposed to insults that a man of spirit could not brook, and which his position prevented him from resenting. If thus treated within the army, he might infer what awaited him when he should emerge from the out-posts of the camp.’ In these circumstances, he came to the desperate resolution of leaving his native country. ‘I cannot any longer,’ he writes to his father-in-law on the 14th of August 1775, ‘bear the insults that are daily offered to me. I cannot bear to be looked upon and treated as the Achan of society. I have done nothing that can deserve this cruel usage. And notwithstanding I have the tenderest regard for my wife and family, and really believe I have an equal return of love and affection from them, though I feel the keenest distress at the thoughts of what Mrs. Thompson and my parents and friends will suffer on my account, and though I foresee and realize the distress, poverty, and wretchedness that must attend my pilgrimage in unknown lands, destitute of fortune, friends, and acquaintances, yet all these evils appear to me more tolerable than the treatment which I meet with at the hands of my ungrateful countrymen.’
Two months after writing the above, he carried his resolution into effect. Paying off his debts, and converting some of his property into cash, with the expressed intention of removing to some of the southern states, where he might live in greater security, he set out from Cambridge, the headquarters of the American army, on the 10th of October 1775, accompanied by his half-brother, Josiah Pierce, who took leave of him at the nearest post-town. ‘From that hour,’ says his biographer, ‘until the close of the revolutionary struggle, his friends and relatives were without any positive tidings of his fate.’ From accounts afterwards received, it appeared that he had reached Newport on the 11th of October, apparently undecided as to his future movements; that there finding a boat belonging to the British frigate Scarborough, he went on board that vessel, and was afterwards landed at Boston, which his friend General Gage, as commander of the British garrison, was at that time maintaining against the American forces. Here he remained under the protection of the British till the evacuation of the town in March 1776, when he again embarked on board the Scarborough, and set sail for England, the bearer of despatches from General Gage to Lord George Germain, the British secretary of state for colonial affairs. Thus had he fairly renounced all connexion with his native country, and gone to push his fortunes in the old world.
Arriving in England, as he did, the bearer of gloomy despatches, and sustaining the equivocal character of a deserter from the American cause, Thompson soon proved that he was a man who could command his fortune anywhere. The capacity in which he had come over introduced him to various public men, who could not fail to be struck by his abilities, as well as charmed by his manner; and the consequence was, that in a short time after his arrival he was offered a post in the colonial office. Probably the minister was of opinion that none of all the American refugees, who then swarmed in London, was able to render such assistance as Thompson in conducting the department over which he presided.
Of whatever nature were the services which Thompson rendered to the public business, they must have been of considerable value; for in 1780, four years after his arrival in England, he was raised by his patron, Lord Germain, to the post of under-secretary of state for the colonies; an instance of promotion which, considering the circumstances in which the subject of it stood, is almost unexampled. The usual accompaniment of such a situation was, and is, a seat in parliament; and according to the practice of those days, when noblemen had seats in the House of Commons at their disposal, Lord Germain, if he had so chosen, might have conferred a seat on his American protege; but it was probably imagined that the admission into parliament of a man so unpopular in America would be attended with disadvantages, and that, at all events, Thompson’s talents were better fitted for the desk than the senate. The income and consequence, however, which he derived from his office, gave him admission to the highest metropolitan circles; and he had thus opportunities not only of becoming known, but also of exercising his inventive mind in many pursuits not immediately connected with his official duties. Fertility—a disposition to propose improvements in all departments—seems to have been his most striking characteristic; and it was probably this ready genius for practical reforms in everything which came under his notice, that recommended him so much to public men. A man who, in his general intercourse with society, can drop valuable suggestions, allowing others to grasp at them, and enjoy the credit of carrying them into effect, is likely to be a favorite. Thompson appears to have been such a man—a person who, holding no ostensible post but that of under-secretary for the colonies, could yet, out of the richness of an ever-inventive mind, scatter hints which would be thankfully received by men of all professions.
While concerning himself generally, however, in a variety of matters, Thompson was at the same time following out certain specific lines of scientific investigation. ‘As early as 1777,’ says his biographer, ‘he made some curious and interesting experiments on the strength of solid bodies. These were never published, and would probably have been superseded by more full investigations made by subsequent experiments. In 1778, he employed himself in experiments on the strength of gunpowder, and the velocity of military projectiles, and these were followed by a cruise of some months in the channel fleet, where he proposed to repeat his investigations on a larger scale.’ On this subject Thompson communicated several papers to the ‘Philosophical Transactions’ of the Royal Society, of which he had become a member. Passing over these scientific lucubrations, we hasten to reach that period of Rumford’s life at which he found himself in a situation to give full scope to his genius for improvements.
As the war between great Britain and the colonies proceeded, it became evident that the latter must triumph. The anti-American party in Great Britain lost ground; and on the news of the capitulation of Lord Cornwallis reaching England, a division took place in the cabinet, and Lord George Germain found it necessary to resign office. As his policy, however, in American affairs, had been agreeable to the wishes of George III, he retired with the honors of a peerage, and was able still to forward the interests of his friends. Not the least distinguished of these was Under-Secretary Thompson, who, whether he had cooperated with his principal in all his measures and views, or whether, ‘according to his own statement afterwards to Cuvier, he was disgusted at Lord Germain’s want of judgment,’ had at least done a sufficient amount of work to deserve a parting token of regard. Accordingly, by the influence of the fallen minister, Thompson was sent out to New York, in the year 1781, with the royal commission of major, which was afterwards changed for that of lieutenant-colonel, charged with the task of organizing an efficient regiment of dragoons out of the broken and disjointed native cavalry regiments which had been fighting on the royalist side. What were to be the specific uses of this force are now uncertain. The regiment, fortunately, was of no avail.
Peace having been concluded between the United States and Great Britain, Colonel Thompson, shortly after his return obtained leave of absence in order that he might travel on the continent.