Defcription of a new Hygrometer and Photometer. 467 



a glafs cafe. But this cafe ferves alfo an important purpofe, for, by confining the circu- 

 lation of the ambient air which alone transfers the continual augmentation of heat, it 

 doubles the performance of the inllrument. The cylindrical cafe fliould be made of clear 

 gl^fs neatly rounded over, and hermetically fealed at the end. Its width is not of much 

 importance, only it fhould leave a free fpace not lefs than Vo of an inch round the balls, 

 and at lead half an inch at the top. Indeed both the fize and form may be regulated by 

 convenience, for I found a receiver of 2200 inches to afford quantities fcarcely one-tenth 

 lefs than thofe given by a cafe of the ordinary dimenfions. 



Since I firft conftruded this inftrument, in the autumn of 1797, I have been delighted 

 with the nicety of its performance. It not only meafures the diredl: rays of the fun, but 

 the refleftcd light of the fky, for which it is principally defigned. It is fenfible to every 

 flu£tuation of the atmofphere, marks the progrefs and decline of the light of day, and the 

 periodic increafe and diminution of the brightnefs of the year. It enables us likewife to 

 eftimate other lights, fuch as the flame of a candle. By comparing two photometers, it is 

 eafy to determine the relative properties of different coloured fubftances — in reflefting, 

 abforbing, and tranfmitting light. In the fame manner, they will determine the queftion, 

 whether the particles of light are fpread over the prifmatic fpe£trum with equal intenfity. 

 By help of this inftrument, too, we can meafure the quantity of light tranfmitted through 

 various diaphanous bodies, and that reflected or abforbed at different angles of incidence 

 from polifhed or rough furfaces ; — in fliort, perform with the utmoft facility all thofe in- 

 genious experiments which have exercifed the fagacity of Bouguer and Lambert. Another 

 fet of inquiries for which the photometer is nicely calculated, is to difcover the condudUng 

 powers of different fluids for heat. If the glafs cafe, for inftance, be filled with a gas of 

 higher condu£ling power than common air, the inftrument will be proportionally lefs af- 

 fe£led by the fame afflux of light, fince thofe are the two balancing conditions. With air, 

 too, of different denfities, the effedts are materially different. In that way I have exa- 

 mined various liquids and gafes, nay jellies and ice. My experiments on thefe and other 

 points are nearly completed, and afford refults which are fatisfaftory and important.* 



JOHN LESLIE. 



Defcription oj the Hydrojlalic Lamp of Mr. Peter Keir. (W. N. ) 



JLT has at all times been confidered as a moft defireable objedl, to afford a conftant and 

 equable fupply of oil to the wick in lamps, and it is an objeft or condition no lefs eflential 

 to the oeconomy of this ufeful inftrument, that ihe light from the flame fliould be inter- 



• This paper was drawn up at Hamburgh in the beginning of laft July, for the purpofe of infertion in 

 the foreign fcientific journals. Previous to its appearance in Englilh, I have revifed it, and made fome 

 flight alterations and additions. 



ccpted 



