248 FIKEj COOKlNGj AND VESSELS. 



Strepsiades. Suppose I take this, and wlien the clerk enters the suit, I 

 stand thus, a long way off, towards the sun, and melt out the letters ! 

 Socrates. Very clever, by the Graces ! "' 



At a mucli later period Pliny mentions that glass balls with 

 water put into them^ when set opposite to the sun, get so hot 

 as to set clothes on fire; and that he finds surgeons consider 

 the best means of cautery to be a crystal ball placed opposite 

 to the sun's rays.^ The Chinese commonl}^ use the burning- 

 lens to light fire with, as well as the flint and steel. ^ 



The fact that fire may be produced by reflecting the sun's 

 rays with mirrors was known as early as Pliny's time (a.d. 

 23-79), as he remarks, " seeing that concave mirrors placed 

 opposite to the sun's rays ignite things more easily than any 

 other fire."* There is some reason to suppose that the know- 

 ledge of this phenomenon worked backwards into history, at- 

 taching itself to two famous names of old times, Archimedes 

 and Numa Pompilius. The story of Archimedes setting the 

 fleet on fire at Syracuse with burning mirrors, probably un- 

 known as it was to historians for centuries after his time, need 

 not be further remarked on here ; but the story of Numa re- 

 appears on the other side of the world, under circumstances 

 which make its discussion a matter of importance to ethno- 

 graphy. 



It is related by Plutarch in his life of Numa, written in the 

 first century, that among the ordinances made for the Vestal 

 Virgins when they were established in Rome, there was the 

 following. If the sacred fire which it was their duty to keep 

 continually burning should happen to go out, it was not to be 

 lighted again from another fire, but new fire was to be made 

 by lighting from the sun a pure and unde filed flame. "And 

 they kindle it especially with vessels which are shaped hollow 

 from the side of an isosceles triangle with a (vertical) right 

 angle, and converge from the circumference to a single centre. 

 When such an instrument is set opposite to the sun, so that 

 the impinging rays from all sides crowd and fold together 

 round the centre, it divides the rarefied air, and quickly kindles 



1 Aristoph., Nubes, 757, etc. = Pliny, xsxvi. 67, ixxvii. 10. 



3 Davis, vol. iii. p. 51. ^ Pliny, ii. 111. 



