pO rvROMETER or PLATIKA. 



The arm of the bended lever, which prefl'es agalnft the end 

 of (his rod, is 25 millimetres in length ; (rather 2f , or about 

 one ninth of an inch) and the arm at right angles to it, or the 

 index, which traverfes on the graduated arc, is 50 millimetres 

 in length (one inch and eight tenths) or twenty tiroes the length 

 of the other. The fpace traverfed by the difplacing of the 

 fmall arm will be thus encreafed in the proportion of 1 to 20. 



As the long arm or indt^x carries a nonius which divides 

 each degree on the graduated arc int(^ ten parts, we can dif- 

 tinftly obferve the 200th of one of thole meafures {referred tQ 

 the bar It felf.) 



Laflly, As the decimal divifion of an arc of a circle of 50 

 millimetres radius, gives only 7.8538 deci-millimetres for one 

 of its degrees, it is evident then that we may meafure an ex- 

 panfion of 0.078538 deci-millimetres, or of the 5730th part 

 of the length of the radius. 



In order to prevent the pofition of the index from being 

 changed in removing the ir-ftrument from the furnace, a plate 

 of platina is fixed fo as to form a fpring againfl: its extremity. 



The author has commenced a feries of experiments to de- 

 termine the range of this pyrometer, to compare.it with the 

 pyromctric pieces of Wedgewood, and fo to fliew the degree, 

 of conlidence it merits, the methods of ufing, and the cafes 

 in which it may be ufefully employed in philofophical refearches 

 and in the arts. 



VIII. 



Letter frotn Mr. Ezekiel Walker on the Froporlion of Light 

 afforded hij Candles of different Dimenfiom. 



To Mr. NICHOLSON, 

 SIR, 



Obfervation"; on X OUR correfpondcnt, who has made fome remarks on my 

 ecitor? " '^^ <^'^Per'ments on candles, does not feem to have fufficiently 

 confidered his fubjed ; for had he paid attention to my paper 

 on page 4-0 of the fourth volume of your Journal, he could not 

 have advanced that " Though Mr. Walker aflerts with con^ 

 fiderable decifion, that the light afforded by candles, is pro- 

 portioned to the quantity of material cynfumed, yet he has not 



given 



