LIB. II. i2. 



157 



comburentia, fortissimo et optirne fabricata, radii lunse 

 possint excipi et colligi in aliquem vel minimum gra- 

 duni teporis 45 . Is vero graclus teporis, si fortasse nimis 

 subtilis et debilis fuerit, ut ad tactum percipi et clepre- 

 hendi non possit ; confugiendum erit ad vitra ilia, quse 

 indicant constitutionem aeris calidam aut frigidam 40 ; 

 ita ut radii lunse per speculum comburens incidant et 

 jaciantur in summitatem vitri hujusmodi ; atque turn 

 notetur, si fiat depressio aqua? per teporem. 



Ad secundam. 



6. Practicetur etiam vitrum comburens super cali- 

 dum, quod non sit radiosum aut luminosum ; ut ferri, 

 et lapidis calefacti, sed non igniti ; aut aquae ferventis, 

 aut similium : et notetur, utrum fiat augmentum et 

 intentio calidi, ut in radiis solis 47 . 



4: &amp;gt; This experiment has been per 

 formed as follows : Tschirnhausen 

 made a lens of power sufficient to 

 fuse copper, silver, tiles, pumice, 

 and even a crucible ; but the rays 

 of the full moon at her greatest alti 

 tude produced no perceptible degree 

 of Heat. In 1802 the lunar rays 

 were again concentrated by the most 

 powerful lens ever made, in the 

 presence of Sir Joseph Banks and 

 other Members of the Royal So 

 ciety; but though the most sensi 

 tive Thermometers were applied, it 

 was thought that a diminution ra 

 ther than an increase of Heat took 

 place. Encycl. Brit. &quot; Burning- 

 glass.&quot; 



46 &quot; Vitrum Calendare.&quot; Cf. infr. 

 II. 13. No. 38. It served also as a 

 barometer, but was naturally rude 

 and inaccurate. For a history of 

 the Thermometer see Herschel s 

 Disc, on Nat. Phil. 356. 



47 See the Encycl. Brit. Article 

 &quot; Burning Glass.&quot; The questions 



started here and in the next para 

 graph have been settled ; and the 

 result is such as proves the simi 

 larity in kind of Solar and other 

 Heat; so that in these places no 

 Negative can be adduced. Large 

 lenses brought before lighted can 

 dles or the fire produce sensible 

 Heat ; and by collecting the rays of 

 Heat in the atmosphere, even when 

 the Sun is quite hidden behind 

 clouds, concave Mirrors produce a 

 definite increase of Heat at their 

 focus. Farther than this, M. M. 

 Saussure and Pictet of Geneva have 

 established the truth as to &quot; ferrum, 

 vel lapis calefactus, sed non igni- 

 tus,&quot; &c. by heating an iron ball 

 so as not to be luminous, and by 

 experimenting on boiling water. 

 From the results they have shewn 

 that Heat emanates in invisible 

 rays, and is subject to the same 

 laws of reflection, &c. as if it were 

 accompanied by rays of light as 

 well. Melloir has also established 



