Variout Methods of Illumtnathn. 67 



afcertaining the rate of flexure from time to time during the filing, by means of the fimple 

 apparatus. Fig. 2. 



I 



III. 



Olft-rvations and Experiments on the Light, Expence and ConJlruBion of Lamps and Candles, 

 and the Probability of rendering Tallow a Suhjlitute for Wax. 



F we difttibute the catalogue of human wants in the order of the necefTity of each, food 

 will occupy the firft place ; and, next to this, in moft climates of the earth, the articles of 

 fuel, lodging, and clothing will immediately prefent themfelves. The total want of any of 

 thefe necelTarily implies extreme diftrefs ; and if fuch a privation be applied even in fancy 

 to an entire fociety, every notion of comfort and civilization feems at once to difappear. 

 Inferior only to thefe in its urgency is the neceffity of artificial light during the abfence of 

 the fun. We might indeed exift without it ; but how large a portion of our lives would 

 in that cafe be condemned to a ftate little fuperior in efficacy to that of the animals around 

 us ! It might be a curious fpeculation to enquire how far and in what refpefts the morals 

 of man would become degraded by the want of fo important an art; fince every ufeful art 

 muft furely in its confequences afteft the moral principles of fociety. But it is fufficient 

 on the prefent occafion that, previous to entering upon a dilTertation refpeiSing the art of 

 illumination, a train of ideas has (lightly been pointed out, which cannot fail to {hew its 

 magnitude and importance. 



We are acquainted with no means, unlefs we may except eledricity, of producing light 

 but by combuftion, and this is moft probably of the fame nature. The rude method of 

 illumination confifts in fucceffively burning certain mafles of fuel in the folid ftate. Com- 

 mon fires anfwer this purpofe in the apartments of houfes, and in fome light-houfes : fmall 

 pieces of refinous wood, and the bituminous coal called kannel-coal, are in .fome coun- 

 tries applied to the fame ufe ; but the moft general and ufeful method is that in which fat 

 oil, of an animal or vegetable kind, is burned by means of a wick. Thefe inftruments of 

 illumination are either lamps or candles. In the lamp, the oil muft be one of thofe which 

 retains its fluidity in the ordinary temperature of the atmofphere. The candle is formed 

 of an oil, or other material, which is not fufible but at a temperature conGderably elevated. 



The method of meafuring the comparative intenfities of light is one of the firft requifites 

 in an enquiry concerning the art of illumination. Two methods of confiderable accuracy 

 are defcribed in the Traite d'Optique of Bouguer, of which an abridged account is given 

 by Dr. Prieftley in his Optics *, page 540. The firft of thefe two methods has been ufcd 

 by others fmce that time, and probably before, from its very obvious nature, but particu- 

 larly by Count Rumford, who has given a defcription and drawings of an inftrument 

 called the Photometer, in the Philofophical Tranfadlions for 1794. The principle it is 

 grounded upon is, that if two lights ftiine upon the fame furface at equal obliquities, and 

 an opake body be interpofcd, the two ftiadows it will produce muft differ in blacknefs or 

 intenfuy in the fame degree. For tlie ftiadow formed by intercepting the greater light 



• The title of this well-known work is : Th/- Ihjlory aiitl prrjeni Slate of Difiovirics rclalii:^ lo yifion. Light, 

 tiij Colours. By Jofcph Pricftlcyi LL. D. F.R. S. London. Johnfun, 1771. 



K 2 will 



