18 CALORIC, HEAT, FIRE. 



who attempted this with any success; he so multiplied the 

 sun's heat by means of mirrors, as to set fire to the Roman 

 fleet at Syracuse. As one plane mirror, for instance, 

 reflects a certain portion of the sun's heat, so two, or three, 

 or any other number, will proportionally increase it if 

 directed to the same object. Leonard Digges, who lived 

 in the reign of Elizabeth, asserts, that by a combination 

 of mirrors he fired bodies half a mile distant. Buffon, 

 by a combination of forty plain glass mirrors, of six 

 inches by fight, set on fire a tarred beach plank sixty-six 

 feet distant ; and with a combination of one hundred 

 and sixty-eight, he performed the same at the distance 

 of two hundred feet. He is said to have melted all the 

 metals at thirty or forty feet. Concave mirrors con- 

 centrate or condense the sun's heat by reflection into a 

 focus, and the effect is increased in proportion as the 

 surface of the mirror exceeds that of the focus. A con- 

 cave mirror of four feet in diameter, made of a mixture 

 of copper and tin, melted a sixpence in less than eight 

 seconds, and a half-penny in twenty seconds. Convex 

 lenses, also, by concentrating the sun's heat into a focus, 

 increase the heat in a similar proportion to concave mir- 

 rors. One of the largest and most powerful lenses of 

 this nature was made some years since by a Mr. Parker, 

 of Fleet-street ; it was formed of flint glass, and when 

 fixed in its frame exposed a clear surface of two feet 

 eight inches and a half in diameter ; its weight was 212 Ibs ; 

 its focal length six feet eight inches, and the diameter of 

 the focus one inch : a second lens was used to receive the 

 focus of the first, which reduced it to half an inch. By 

 these lenses the sun's rays were condensed in the ratio 

 of four thousand two hundred and twenty-five to one, and 

 the effects produced were astonishing. Iron, when ex- 

 posed to the focus, melted in a moment ; slates, tiles, 

 &c., became red hot, and vitrified instantly ; and wood, 

 acted on under water, became burnt to a coal. Mr. 

 Parker had the curiosity to try what the sensation of 

 burning at the focus was, and having passed his finger 

 through it, he described it to be like that of a cut with 

 a lancet. 



Caloric is also produced by combustion, through the 



