Instruments of the Antients. 239 



of these last were sufficient to produce fire. Vitcillo assures 

 us of this fact, in the fifth hook of his optics. This pas- 

 sage, then, proves, that the invention is at least 1200 yeai* 

 old. 



71. " This antiquity will he increased by 600 years, \i Ar- 

 chimedes really used plane mirrors to set on fire the Roman 

 fleet at the siege of Syracuse. Although this circumstance 

 cannot be directly proved by any positive passage in any an- 

 tient author; yet, from the words of Tzetzes, when speak- 

 ing of this fact, we may draw arguments which will enable 

 us to conjecture with much probability. For he says pre- 

 cisely, that the apparatus of Archimedes was composed of 

 many mirrors, and that those mirrors were moveable, by 

 means of certain planks. 



72. (i On this circumstance, I observe in the first place, 

 that although a compound burning mirror may be formed of 

 concave mirrors, as well as of plane ones, while it does not 

 appear that Archimedes used the one rather than the other; 

 yet we have at least as good a right to say, that his mirrors 

 were plane, as that they were concave. A machine is more 

 simple and more easily executed with plane mirrors than 

 with concave ones : it is therefore more natural to think, 

 that Archimedes composed his machine of the former than 

 of the latter. 



73. (£ Add to this, that to compose a burning mirror of 

 concaves, it is by no means necessary that they be moveable; 

 though this property be of great utility if the component 

 mirrors be plane. For the making concave mirrors move- 

 able will not alter their common focus, or carry it to many 

 different distances, and consequently by their means flame 

 cannot be produced at more than a determinate distance. 

 Hence it follows, that when the burning point was to be 

 carried either nearer or further off, a new and totally diffe- 

 rent machine would have been necessary, which could not 

 but be attended with great difficulties in practice. 



74. ie But if the machine were composed of plane mirrors, 

 all these difficulties would disappear: even inconveniences 

 would vanish; the burning point might have been carried 

 to very different distances, greater or less, as might have 

 been required, merely by a greateror less inclination to each 

 of the plane mirrors. 



75. " The object of Archimedes was to produce fire at a 

 great distance. For this end, it was necessary for him to con- 

 struct it in such a manner, that he might easily uiake the 

 burning point fall exactly on the Roman ships, to which he 

 wished to set fire. The distance of those ships was always 



uncertain, 



