[18] The preceding paragraphs, and some other parts of the present volume on the general properties of Heat, are taken from my Treatise on Heat, in the _Cabinet Cyclopœdia_, to which those who desire more detailed explanation and more copious illustration should refer. [Pg119] [Illustration: GLASGOW COLLEGE.] CHAP. V. WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE WITH A DUE ECONOMY OF FUEL. — CONCEIVES THE NOTION OF CONDENSING OUT OF THE CYLINDER. — DISCOVERS SEPARATE CONDENSATION. — INVENTS THE AIR-PUMP. — SUBSTITUTES STEAM PRESSURE FOR ATMOSPHERIC PRESSURE. — INVENTS THE STEAM CASE, OR JACKET. — HIS FIRST EXPERIMENTS TO REALISE THESE INVENTIONS. — HIS EXPERIMENTAL APPARATUS. — DIFFICULTIES OF BRINGING THE IMPROVED ENGINES INTO USE. — WATT PRACTISES AS A CIVIL ENGINEER. — HIS PARTNERSHIP WITH ROEBUCK. — HIS FIRST PATENT. — DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE. (68.) At the period to which we have now brought the history of the invention of the steam engine, Watt had obtained, chiefly by his own experiments, a sufficient knowledge of the phenomena which have been just explained, to enable him to arrive at the conclusion that a very small proportion of the whole mechanical effect attending the evaporation was really rendered available by the atmospheric engine; and that, [Pg120] therefore, extensive and injurious sources of waste existed in its machinery. He perceived that the principal source of this wasteful expenditure of power consisted in the quantity of steam which was condensed at each stroke of the piston, in heating the cylinder previous to the ascent of the piston. Yet, as it was evident that that ascent could not be accomplished in a cold cylinder, it was apparent that this waste of power must be inevitable, unless some expedient could be devised, by which _a vacuum could be maintained in the cylinder, without cooling it_. But, to produce such a vacuum, the steam must be condensed; and, to condense the steam, its temperature must be lowered to such a point that the vapour proceeding from it shall have no injurious pressure; yet, if condensed steam be contained in a cylinder at a high temperature, it will return to the temperature of the cylinder, recover its elasticity, and resist the descent of the piston.