The scientific work which Sir B. Thompson did whilst in the service of the Elector of Bavaria between 1783 and 1794, shows his energy and originality, his accuracy and his depth.
When at Mannheim in July 1785 he made experiments in the presence of Professor Hemmer, of the Electoral Academy of Sciences of Mannheim, on the propagation of heat through various substances; on the increased difficulty of conduction of heat through the torricellian vacuum; on the effect of humidity in increasing the conducting power of the air; and on the effect of air of different degrees of density. The Duke ordered the meteorological instrument maker to the academy at Mannheim to come to Munich, and to spare neither labour nor expense in providing the complete apparatus necessary for the experiments.
These experiments, on the relative conducting powers of mercury, water, air, and a torricellian vacuum, were read to the Royal Society, March 9, 1786.
He then proceeded to make experiments on the relative warmth of various substances used in making artificial clothing; relative quantities of the same substance; different qualities of substance chemically, as charcoal, ashes, dust. All his experiments indicated that the air which occupies the interstices of substances used in forming coverings for confining heat acts a very important part in that operation. Air is a perfect non-conductor of heat. These experiments were chiefly made in 1787. They were not read before the Royal Society until January 19, 1792.
Early in the winter of 1787, as soon as the cold was sufficiently intense, he began to repeat the experiments of Dr. Fordyce (‘Transactions of the Royal Society,’ vol. lxxv.) on the weight said to be acquired by water in the act of freezing; and, being possessed of a most excellent balance belonging to the Duke of Bavaria, he soon came to the conclusion that all attempts to discover any effect of heat upon the apparent weights of bodies would be fruitless.
He had previously, in April 1785, convinced himself of the errors that arose from currents of air and from the drying of the cords by which the scales were hung.
These experiments were made into a paper entitled ‘An Inquiry concerning the Weight Ascribed to Heat.’ This was read before the Royal Society, May 2, 1799.
In May and June 1786 he made experiments on the production of air from water exposed to light. These were read before the Royal Society, February 15, 1787.
When engaged in his experiments on the conducting powers of various bodies with respect to heat, and particularly of such substances as are used for clothing, he made experiments on the relation between their conducting power and their power of absorbing moisture, but found none. Flannel and fur, contrary to his expectation, absorbed much more moisture from the air than silk and cotton. On this he forms an idea of the good of wearing flannel. This, the weakest of his papers, was read to the Royal Society, March 22, 1787.
In the spring of 1791 a large building was erected in the neighbourhood of Munich, on the ground destined for the exercise of the artillery, where a most complete apparatus was put up for measuring the velocities of cannon bullets by the recoil of the gun, and also by the pendulum at the same time, and with this apparatus a great number of interesting experiments were made.