METHOD OF INCREASING THE LIGHT OF CANDLES. £75 



general titles of Wax, Spermaceti, Tallow, and mixed ; and 

 tabulated, to (hew, 1 . Average weight of the wick ; 2. Num- 

 ber of threads ; 3. Weight of the whole candle ; 4. Diameter ; 

 5. Length; 6. Time of combuftion per inch; 7. Ditto per 

 ounce; 8. Quantity or intenfity of light when newly muffed ; 

 9. Ditto after burning half a minute, or till the ufual period of 

 fnuffing ; 10. Average intenfity ; 11. Expence of material per 

 hour to produce the quantity 1.00 of light; 12. Ditto in the 

 money price of the a/ticle; 13. Station of the barometer; 14. 

 Ditto of the thermometer; 15. And ditto of the eudiometer. 

 I am abfolutely unacquainted with any good ilandard of com-. Stan(Jar( * jj£j**J 

 parifon of light. The fun perpetually varies with the weather, un k nown . 

 its altitude, &c. It feems probable that an earthen lamp, 

 having a wick of known texture and weight, cut to a deter- 

 minate elevation by a gage, and charged with very pure olive 

 oil, might, at like ilations of the barometer and thermometer, 

 give light fo nearly equal, for the firfl two or three hours, as to 

 afford refults confiderably exact ; of which the means of feveral 

 repetitions would be of great practical utility. The com pari- Method of com^ 

 fon of lights by the fhadows has been often mentioned ; but fljgffSLfi^ 1 by 

 fimplicity and value of the method (not yet univerfally known) 

 will juftify a fhort repetition. When the fliadows of the fame 

 obje6l fall upon each other, not precifely, the bordering fliades 

 will be feverally illuminated by one light only: move the 

 lights till thefe bodies appear equally dark at the point or place 

 where tbey-meet in an angle. Then meafure the diflances 

 of the lights from that point or place, and the intenfities are 

 inverfely as the fquares of thofe diflances. It is to be noticed 

 that the obftacle which forms the fhadow, a book for example, 

 mufl be held fo that the ray from each light may fall on the 

 wall nearly in the fame angle ; and this will always be the cafe 

 if the farther light be nearly behind the other. 



In a fubfequent letter, Mr. Walker informs me, that he care- 

 fully meafured his intenfities by the method of fhadows, as 

 mentioned in Prieftley's Optics. I conclude this to have been 

 the method here defcribed ; though, for want of a good index, 

 I can only find the method of Bouguer in that work. 



T 2 IX. Defcription 



