30' Mechanical Philosophic, 



different intensity, and the variety of tones of 

 sound, in different gases, by Priestley, CnLADNiy 

 Jacquin, Perolle, and others. 



OPTICS. 



• In this science great improvements have takeri' 

 place in modern times. In 1704 Sip Isaac New- 

 ton first published his grand work oiti Optics; and 

 although many of his most interesting discoveries 

 were made and announced toward the close of the 

 seventeenth century, yet the collection and pub- 

 lication of them, in a systematic form, was re- 

 served to be one of the distinguishing honours of the 

 eighteenth. How numerous and important these 

 discoveries were, is generally known. He ascer- 

 tained the different refrangibility of the rays of 

 light; he made some progress in exploring the 

 principles and laws of colours, which had been so 

 little understood before his time ; he first explained 

 the physical cause, and laid down with mathema- 

 tical precision, the general laws of the reflection 

 and refraction of light ; besides many other valua- 

 ble, but less important additions to the science of 

 optics. It must be acknowledged that his doc- 

 trines are by no means free from errors and defects ; 

 but these are few in comparison of their great 

 merits; and have been chiefly corrected or sup- 

 plied by the labours of subsequent philosophers. 



Since the discoveries of Newton many import- 

 ant additions have been made to our knowledge 

 of the nature and properties of light. The mate- 

 riality of this substance, and the great vclocitij of 

 its motion, were more fully illustrated and con- 

 firmed than they had been before, by Dr. Brad- 

 ley and Mr. Molyneux, in 1727. A few years 

 afterwards M. Bouguer, a celebrated French phi- 

 losopher;> distinguished himself by his experiments- 



