CHAP, iv.] APPLICATIONS OF THE LA WS OF HKAT. 365 



CHAPTER IV. 



VARIOUS APPLICATIONS OF THE LAWS OF HEAT. 



I BURNING GLASSES AND MIRRORS. 



Is it true that Archimedes, by the use of burning glasses, set fire to 

 the Eotnan fleet, which, under the command of Marcellus, was be- 

 sieging Syracuse ? Is it true that Proclus did as much to the fleet of 

 Vitellius during the siege of Byzantium ? 



These are questions which have been much discussed which 

 Descartes in his Dioptrique has answered in the negative which the 

 learned have solved in various ways, but which prove at least that the 

 ancients were acquainted with the power of concave mirrors to reflect 

 to their focus and condense into a very small space, the rays which 

 emanated from a source of heat. 



They were aware also of the effects of refraction through a mass of 

 glass shape in the form of a ball or a lens, as is deduced from a very 

 curious passage in the Clouds of Aristophanes. 



The discussion of this point of history, interesting as it is from 

 other points of view, has had the merit of inducing experiments 

 which have placed beyond doubt the heating effects which may be 

 produced by bringing the rays of the sun to the focus of a spherical 

 or parabolic mirror, or to that of one or more lenses. The following 

 are the principal results of some of these experiments recorded in 

 D'Alembert and Diderot's Encyclopaedia : 



" The most celebrated burning mirrors of modern times are those 

 of Septala, Vilette, and Tschirnhausen. The burning mirror of Manfred 

 Septala, canon of Milan, was a parabolic mirror which, according to 

 Schot, could set fire to pieces of wood at a distance of fifteen or 

 sixteen paces. The burning mirror of Tschirnhausen was at least equal 



