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be reflected npon it in an oblique, or even opposite 

 direction to that in which they came from the sun 

 itself. And he adds, " that the assigned distance 

 being so considerable, it might appear at first im- 

 possible to effect this by means of the reflection of 

 the sun's rays, but as the glory Archimedes had 

 gained by thus setting fire to the Roman vessels, was 

 a fact universally agreed in, he thought it reasonable 

 to admit the possibility of it, upon the principle he 

 had laid down." He afterwards advances farther in 

 this enquiry, establishing certain necessary propo- 

 sitions in order to come at a solution of it. "To 

 find out therefore in what position a plain mirror 

 should be placed to carry the sun's rays by reflection 

 to a given point, ho demonstrates that the angle of 

 Incidence, is equal to the angle of reflection ; and 

 having shewn that in so just a position of the 

 glass, the sun's rays might be reflected to the giren 

 place, he observes, that by means of a number of 

 glasses, reflecting the rays into the same focus, there 

 must arise at the given place the conflagration re 

 quired, for inflaming heat is the result of thus con- 

 centrating fie sun's rays ; and that when a body is 

 thus set on fire, it kindles the air around it, so that 

 it comes to bo acted upon by the two forces at once, 

 that of the sun, and that of the circum -ambient air, 

 reciprocally augmenting and increasing the heat :" 

 whence, continues he, " it necessarily results, that by 

 a proper number of plain mirrors duly disposed, the 

 sun's ray might he reflected in such quantity into a 

 common focus at a bo\v shot distance, as to set all ia 

 flames around it." 



5. cc As to the manner of putting this in prac. 

 tice," he says, " it might be done by employing many 

 hands to hold the mirrors in the described position ; 

 but to avoid the confusion that might thence arise, 

 twenty-four mirrors at least being requisite to commu- 

 nicate flame at such a distance, he fixes upon another 

 method, that of a plain hexagon mirror ; accoinmodated 



