66 ON HI! AT. 



Sect. I. Short Account of a new Experiment on Heat. 



I have lately made a new experiment, the refult of which 



appears to me fuiliciently interefting to deferve the attention 



of the cla(#. 



Qa. Whether * Having found by experiments often repeated that metallic 



the heating and bodies Cxpofed in the free air of a large apartment are much 



ed°an<fof bbek- morefpeedily heated and cooled when their furfaces have been 



ened bodies fbl- blackened (over the flame of a candle for example) than when 



!n W firuU f c!o 1 fcd aW the >' are clean and P 0,imed » 1 was curIous t0 ^ now whether 

 fpacesasinfpaces the fame phenomena would take place when, inftead of ex- 

 more enlarged ? p f in g t} ie r e bodies in the open air* they mould be placed in 

 clofe metallic veflels furrounded by a certain thicknefs of in* 

 eluded air, and thefe vefTels fhould be then plunged in a large 

 a mafs of hot or cold water. In ojjjjer to clear up this impor- 

 t tant point, I made the following experiment: 



A cylindrical A cylindrical veffel of brafs, three inches in diameter and 



veffel of thin, four inches long was enclofed in another larger cylindrical vef- 

 portetTin the ^ e '» m lne centre of which it was fufpended by its neck, fo as 

 middle of a larger to touch it in no other part, leaving on all fides an interval of 



ta«'A" * . one inch belween ' he veffels - 



ter4l or* air be- The external veffel as well as the fmaller one included with- 

 tween them. j n j t j s mac [ e 5^ fj 1 j n Q iee t s of brafs: its diameter is five inches 

 and its height fix. It is one inch and a half in diameter and 

 fix inches high. Its neck is one inch and a quarter in diame- 

 ter and two. inches and a "half long. 



The interior veffei isfufpended in the centre of the externa! 

 one by a ftopper of cork: This flop per is adjufted to the neck 

 of the external veilei, and there is a cylindrical hole of three 

 quarters of an inch diameter through the cork, and having the 

 fame axis, which perforation receives the neck of the interior 

 veffel and retains it in its place. 



The interior veffel was introduced and fixed in its place be- 

 fore the bottom of the exterior veiiel was (bldered in. 

 The larger veffel - At the centre of the bottom of the great veflel is a fmall me- 



was fupported tallic tube of three quarters of an inch diameter and one inch 

 oa a foot. , J , '-' . r ...... _ . . . 



and a half long, by means or which this inftrument is attached 



to a folid heavy foot of metal which fupports it in a vertical 



pofition when the whole inftrument is fubmerged in a veffel 



of water. 



This inftrument, which greatly refembles that defcribed in 



my 7 th effay on the propagation of heat in fluids, which I have 



called 



