Note 85 — Chapter VIII · Watt’s Connexion with Dr. Roebuck—Watt acts as Surveyor and Engineer.

“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.