36 WATT. 



examination of the model what were the defects of the 

 machine itself, and which no care in repairing or ad- 

 justing that model could remove. He found first of 

 all that the boiler was much too small in proportion to 

 the column of water which the steam had to raise, and 

 yet it was larger than the boiler used in practice. The 

 cylinder was on the scale of two inches diameter, the 

 height being half a foot. The vacuum being imper- 

 fect from the size of the boiler, he diminished the 

 length of the piston-rod. He found that the brass of 

 which the cylinder was made carried off a great deal 

 of heat, and that too large a surface was exposed to the 

 steam. These observations set him upon making a va- 

 riety of experiments upon steam, and upon the mode 

 of applying it both directly and to produce a vacuum. 

 He had, in the year 1759, while a fellow-student with 

 Mr. Robison, received from that gentleman a sugges- 

 tion of the application of steam to wheel-carriages, as he 

 tells us in 1803, long before steam travelling was dreamt 

 of* They had together made experiments on Papin's 

 digester, in order to ascertain the expansive force of 

 steam; but these speculations had for several years 

 been given up. In 1760 and the two following years 

 Watt had been in familiar intercourse with Professor 

 Black, had witnessed his experiments on heat, and 

 had learnt from him the true cause of evaporation 

 and condensation. When, therefore, he began to ex- 

 periment upon the mechanical application of steam, 

 its expansion, and its condensation, he enjoyed that 

 inestimable advantage of thoroughly knowing the 

 principles on which its changes and its action depended. 

 His own experiments now put him in possession of 

 the causes which determine the rapidity of evaporation, 

 the proportion which it bears to the surface exposed 



* Mr. Murdock, in 1784, made a working model of a steam-carriage, 

 which moved about the room. It was constructed upon the principle set 

 forth in Mr. Watt's specification of 1769, Art. iv., and this is the very 

 method used at the present day 



