He could not, however, shake off his recurring fits of despondency, and on his return to Glasgow, we find him occasionally in very low spirits. Though his head was full of his engine, his heart ached with anxiety for his family, who could not be maintained on hope, already so often deferred. The more sanguine Roebuck was elated with the good working of the model, and impatient to bring the invention into practice. He wrote Watt in October, 1768, “You are now letting the most active part of your life insensibly glide away. A day, a moment, ought not to be lost. And you should not suffer your thoughts to be diverted by any other object, or even improvement of this, but only the speediest and most effectual manner of executing an engine of a proper size, according to your present ideas.” Watt, however, felt that his invention was capable of many improvements, and he was never done introducing new expedients. He proceeded, in the intervals of leisure which he could spare from his surveying business, to complete the details of the drawings and specification,--making various trials of pipe-condensers, plate-condensers, and drum-condensers,--contriving steam-jackets to prevent the waste of heat and new methods for securing greater tightness of the piston,--inventing condenser-pumps, oil-pumps, gauge-pumps, exhausting-cylinders, loading-valves, double cylinders, beams, and cranks. All these contrivances had to be thought out and tested, elaborately and painfully, amidst many failures and disappointments; and Dr. Roebuck began to fear that the fresh expedients which were always starting up in Watt’s brain, would endlessly protract the consummation of the invention. Watt, on his part, felt that he could only bring the engine nearer to perfection by never resting satisfied with imperfect devices, and hence he left no means untried to overcome the many practical defects in it of which he was so conscious. Long after, when a noble lord was expressing to him the admiration with which he regarded his great achievement, Watt replied: “The public only look at my success, and not at the intermediate failures and uncouth constructions which have served me as so many steps to climb to the top of the ladder.” As to the lethargy from which Roebuck sought to raise Watt, it was merely the temporary reaction of a mind strained and wearied with long-continued application to a single subject, and from which it seemed to be occasionally on the point of breaking down altogether. To his intimate friends, Watt bemoaned his many failures, his low spirits, his bad health, and his sleepless nights. He wrote to his friend Dr. Small[83] in January, 1769, “I have many things I could talk to you about--much contrived, and little executed. How much would good health and spirits be worth to me!” A month later he wrote, “I am still plagued with headaches, and sometimes heart-aches.”