Steam-Engine-. 421 



piston reached its limit. A good machine ought always to tend 

 itself by means of the first moving power, without any foreign 

 aid. Another great inconvenience was the introduction of steam 

 into a cold cylinder ; since a great loss was thereby occasioned, 

 and was repeated at each stroke of the piston, the cylinder being 

 continually cooled by the injected water necessary for the precip- 

 itation. But these defects, which in the present state of the sci- 

 ences we are so prompt to observe, could not at first have been so 

 easily detected. They were perceived and corrected in 1 764, by 

 Watt, the disciple and friend of Black. Being then at Glasgow, 

 where he was employed as a mathematical instrument maker, he 

 was directed to repair a small model of Newcomen's engine, 

 which belonged to the University in that city. In the course of 

 his attempts to make its operation satisfactory, he observed that 

 it consumed more coal in proportion to its size than the large 

 engines. Being curious to ascertain the cause of this dif- 

 ference, and wishing to remedy so great a defect, Watt made 

 numerous experiments for the purpose of determining what sub- 

 stance is the most suitable for the cylinders ; and what are the 

 most proper means of creating a perfect vacuum ; what is the 

 temperature to which water rises in boiling, under different 

 pressures ; and what the quantity of water necessary to produce 

 a given volume of steam, under the ordinary pressure of the at- 

 mosphere. He determined also the precise quantity of coal 

 necessary to convert a known weight of water into vapour, and 

 the quantity of cold water required to precipitate a given weight 

 of steam. These several points being once exactly ascertained, 

 he was led to perceive the defects of Newcomen's engine, and 

 to assign the cause of these defects. He saw that the steam 

 could not be condensed so as to produce any thing like a va- 

 cuum, unless the cylinder and the water it contained, as well as 

 that injected and that arising from the condensed steam, were 

 cooled down at least to the temperature of about 33, and that 

 at a higher temperature the steam had still an elasticity strong 

 enough to oppose a very perceptible resistance to the weight of 

 the atmosphere. On the other hand, when it is proposed to at- 

 tain more perfect degrees of exhaustion, the requisite quantity of 

 injected water is augmented in a very rapid proportion ; and 

 hence results a great loss of steam, when the cylinder is again 

 filled. These facts led Watt to conclude, that, in order to effect 



