PHYSICS OF THE EIGHTEENTH CENTURY. 295 



the microscopes. Let e, h, g, k be supposed to have been permanently 

 adjusted, and tne object-glass /to have been brought into the position 

 in which the image of e coincides with the-cross wire of the eye-piece. 

 This is accomplished by means of a screw, which gives a longitudinal 

 motion to the whole bar in the middle trough. The object-glass at / 

 can be moved longitudinally along the bar by means of a screw of 

 known thread, that is, the amount of longitudinal motion due to a turn 



FIG. 143. 



of the screw is known, and parts of a turn are indicated on a graduated 

 circle. When all the three bars are in melting ice, this screw is so 

 adjusted that the optical axes of both microscopes coincide with e and 

 h. The middle trough is next heated to 2 1 2, the middle bar is, if ne- 

 cessary, adjusted to the original coincidence at/, and the number of 

 turns which must now be given to the screw at / to bring h into coin- 

 cidence again, indicates the elongation of the bar. 



The principles of latent heat had no sooner been expounded by 

 Black than they found a famous practical application in the hands of the 

 illustrious WATT (1736 1819), whose steam-engine has changed the 

 whole world of arts and manufactures more than any other invention 

 that has ever been made. It is, of course, quite possible that the 

 steam-engine would have been hit upon, even if Black had never 

 enunciated a single proposition concerning latent heat. But we can 

 hardly doubt that Black's doctrine must have guided Watt in his in- 

 vention, Watt was certainly acquainted with the course of physical 

 and chemical instruction given at the University of Glasgow, for he 

 was the officially appointed instrument maker to the university, and in 

 frequent communication with Plack, and with Anderson the professor 

 of physics. The latter had sent him a working model of Newcomen's 

 pumping engine, with a view of having its defective action remedied, 

 if possible. Watt soon rectified the defects, and the apparatus was 



