Watt paid his promised visit in May, 1768, and Roebuck was by this time so much interested in the invention, that the subject of his becoming a partner with Watt, with the object of introducing the engine into general use, was seriously discussed. Watt had been labouring at his invention for several years, contending with many difficulties, but especially with the main difficulty of limited means. He had borrowed considerable sums of money from Dr. Black to enable him to prosecute his experiments, and he felt the debt to hang like a millstone round his neck. Watt was a sickly, fragile man, and a constant sufferer from violent headaches; besides he was by nature timid, desponding, painfully anxious, and easily cast down by failure. Indeed, he was more than once on the point of abandoning his invention in despair. On the other hand, Dr. Roebuck was accustomed to great enterprises, a bold and undaunted man, and disregardful of expense where he saw before him a reasonable prospect of success. His reputation as a practical chemist and philosopher, and his success as the founder of the Prestonpans Chemical Works and of the Carron Iron Works, justified the friends of Watt in thinking that he was of all men the best calculated to help him at this juncture, and hence they sought to bring about a more intimate connection between the two. The result was that Dr. Roebuck eventually became a partner to the extent of two-thirds of the invention, took upon him the debt owing by Watt to Dr. Black amounting to about 1200L., and undertook to find the requisite money to protect the invention by means of a patent. The necessary steps were taken accordingly and the patent right was secured by the beginning of 1769, though the perfecting of his model cost Watt much further anxiety and study. It was necessary for Watt occasionally to reside with Dr. Roebuck at Kinneil House while erecting his first engine there. It had been originally intended to erect it in the neighbouring town of Boroughstoness, but as there might be prying eyes there, and Watt wished to do his work in privacy, determined "not to puff," he at length fixed upon an outhouse still standing, close behind the mansion, by the burnside in the glen, where there was abundance of water and secure privacy. Watt's extreme diffidence was often the subject of remark at Dr. Roebuck's fireside. To the Doctor his anxiety seemed quite painful, and he was very much disposed to despond under apparently trivial difficulties. Roebuck's hopeful nature was his mainstay throughout. Watt himself was ready enough to admit this; for, writing to his friend Dr. Small, he once said, "I have met with many disappointments; and I must have sunk under the burthen of them if I had not been supported by the friendship of Dr. Roebuck."