In preparing his specification, Watt viewed the subject in all its bearings. The production of power by steam is a very large one, but Watt grasped it thoroughly. The insight with which he searched, analysed, arranged, and even provided for future modifications, was the true insight of genius. He seems with an almost prophetic eye to have seen all that steam was capable of accomplishing. This is well illustrated by his early plan of working steam expansively by cutting it off at about half-stroke, thereby greatly economising its use;[85] as well as by his proposal to employ high-pressure steam where cold water could not be used for purposes of condensation.[86] The careful and elaborate manner in which he studied the specification, and the consideration which he gave to each of its various details, are clear from his correspondence with Dr. Small, which is peculiarly interesting, as showing Watt’s mind actively engaged in the very process of invention. At length the necessary specification and drawings were completed and lodged early in 1769,--a year also remarkable as that in which Arkwright took out the patent for his spinning-machine. [85] “I mentioned to you a method of still doubling the effect of the steam, and that tolerably easy, by using the power of steam rushing into a vacuum, at present lost. This would do a little more than double the effect, but it would too much enlarge the vessels to use it all. It is peculiarly applicable to wheel engines, and may supply the want of a condenser where force of steam is only used; for, open one of the steam valves and admit steam, until one-fourth of the distance between it and the next valve is filled with steam, shut the valve, and the steam will continue to expand and to press round the wheel with a diminishing power, ending in one-fourth of its first exertion. The sum of this series you will find greater than one-half, though only one-fourth steam was used. The power will indeed be unequal, but this can be remedied by a fly, or in several other ways.”--Watt to Small, 28th May, 1769. Boulton MSS.