216 HYDRAULICS. 



purpose ; because a skilful engineer can readily 

 find a method of converting the reciprocating mo- 

 tion of the working-beam, into a motion of any 

 kind which may suit his purpose. Savary's engine 

 could not admit of such a general application, and 

 seems almost restricted to raising water. 



Inventions improve by degrees. The most 

 exact and unremitting: attention was at first re- 

 quired to open and shut the cocks precisely at 

 the proper time ; for neglect might be ruinous 

 to the machine, by the confined steam beating 

 out the bottom of the cylinder, or allowing the 

 piston to be wholly drawn out of it. Stops 

 were contrived to prevent these accidents ; then 

 strings were used to connect the handles of 

 the cocks with the beam, so that they should 

 be turned whenever it was in certain posi- 

 tions. These were gradually changed, and im- 

 proved into detents and catches of different shapes ; 

 at last, Mr. Beighton simplified the whole of these 

 subordinate movements. 



About 1762, the late Mr. Watt began to turn 

 his attention to this machine, which he has since 

 brought to so great a degree of perfection. 



But before we explain Mr. Watt's improvements, 

 it is necessary to premise a short account of the im- 

 perfections of the old steam-engines, and their 

 causes. 



The pressure of the atmosphere, or any equi- 

 valent resistance, prevents the production of steam, 

 until the water be heated to 212 degrees of Fah- 

 renheit's thermometer ; but when that pressure is 

 removed, or the water be placed in a vessel ex- 

 hausted of air, steam is produced from it when it 

 is colder than the human blood. On the contrary, 

 if water be pressed upon by air or steam, which 



