Instruments bf ihe Antic nts. 235 



the art of working mirrors, was sufficient to enable them to 

 invent, or to find out by chance, a mirror which could pro- 

 duce such effects ; or whether this kind of knowledge was 

 in such a state among the antients as to put the fabrication 

 of such a mirror beyond the limits of probability, or make 

 it fit to be considered as morally impossible. 



57. i( The antients certainly possessed a great variety of 

 knowledge which has not come down to us ; and, among 

 those parts of their science of which we have still some ves- 

 tiges in the authors of antiquity, some are not sufficiently 

 intelligible for want of attention in the readers, or because 

 they have not been clearly enough explained by the writers, 

 ilence it is that wc discover, from time to time, in those 

 antient writers many things which we have been accus- 

 tomed to consider as new discoveries. Consequently we 

 run much greater risk of being mistaken, by averring that 

 the antients did not possess certain parts of science known 

 to us, than by endeavouring to prove that they did possess 

 them. For the testimony of one antient author is sufficient 

 to prove that they had the knowledge of a particular thing; 

 whereas, to prove the contrary, it would be necessary to 

 consult all the antient authors who may have spoken of 

 that thing, and to be very sure that we rightly understand 

 them. 



58. "" As to catoptrics and dioptrics, I say, that the sci- 

 ence of the antients went somewhat further than is gene- 

 rally believed. Of this assertion I shall give some proofs, 

 and chiefly such as may contribute to give probability to 

 the existence of the mirror of Ptolemy. 



59. " M. cle Fontenelle, in the Hist, of the Acad, of Sc. 

 •for 1708, says, that " burning mirrors were undoubtedly 



known to the antients ; for some historians allege that Ar- 

 chimedes used them in burning a fleet; and, although they 

 attributed to them an impossible effect *, even this circum- 

 stance proves them to have been known. But it is certain 

 that those mirrors which they contrived must have been of 



* The actual production of this " impossible effect," by Bujfcm, in the 

 year 1747 (not to mention what had been done by Leonard Digges and 

 father Kircbcr in the two preceding centuries) proves how very cautious 

 philosophers ought to be in dogmatically applying the epithet impossible 

 to natural effects which they have never seen or experimented. About 

 the same period another impossibility was performed, when Frank/in drew 

 lightning from a thunder cloud. Fontaiclle lived to see both these dis- 

 coveries, and no doubt to be convinced that such language from men of 

 eminence tends exceedingly to retard the progress of knowledge, by dis- 

 couraging the attempts of persons of less celebrity perhaps, but often o£ 

 equal or superior ability. — Translator. 



R 4 metal, 



