1 74 IFright cic. on an Universal Measure. 



and produce the effect of an echo ; for the voice is reflected, 

 from the poHshed and smooth surface of the speculum, more 

 directlv and entirely than from any wall." 



The reflection of heat from a concave mirror is acknow- 

 ledged to be of very remote antiquity. Not so what is called 

 the reflection of cold. This discovery seems to be generally 

 ascribed to one of ou/ cotemporaries on the continent : with 

 what iustice the foregoing, extract shows. The experiment, 

 indeed, was successfully repeated, seventy years after Poita 

 had put it in print, by the Academy del Chnento ; it being 

 the ninth of their Collection of Experiments, published at 

 Florence in 1666. The reflection of sound from concave 

 mirrors, is also a very old discovery. This was most pro- 

 bably the principle of the talking brazen head, which po- 

 pular tradition, m the southern part of this island, ascribes 

 to Rog^er Bacon, in the northern, to Michael Scot, and in 

 foreitrn countries, to other ainning men. It. was no doubt 

 the true secret oh the enchanted head in Don Quixote, and 

 of the 8Sth of the Century of Inventions, published, ahiiost 

 150 years ago, by the Marquis of JVorcestcr, who was un- 

 fortunately regarded by most of his cotemporaries as little 

 superior, in sobriety of mind, to the knight of La Mancha. 



Though I have no time for further remarks, I cannot help 

 asking, Whether, if it be true, as it very probably is, that 

 cold is the mere privation or abstraction of heat, the ex- 

 pression *' reflection of cold," be not an absurdity, both 

 in grannnar and physics ? Is it not like ascribing a positive 

 eftect to a mere negation P or like saying, that all things 

 were made by nothing P Perhaps the best answer which 

 could be made to these queries would be to say. That as we 

 are entirely ignorant of the intimate essences of things, it 

 cannot be expected that our language should always apply 

 with strict propriety to phsenomena which depend on those 

 unknown intimate essences. For physics, I apprehend, are 

 as far from bemg a science, strictly so called, in the present 

 period, as v/hen Locke, above a century ago, gave his rea- 

 sons for " suspecting that Natural Philosophy was not ca- 

 pable of beini;' n)ade a science." See § 10. ch. 12. b. 4. of 

 the Essav on Human Understanding, a work which de- 

 serves the serious attention of such of our present experi- 

 menters as arc fond of being called philosophers and men 

 of science. 



xxviir. A 



