HYGROMETER FOR GASSES. 287 



In the sector of a circle, the angle at the centre is a 



function of the arc and the radius ; viz : Ano^le oe — — — — • 



Radius 



but this is no equation, except we make an arbitrary choice 

 of units. 



Does not alnaost the whole of dynamics consist of equa- 

 tions (or to speak more properly analogies) between hetero' 

 gen^OMS quantitities ? But I imagine the falsehood of this 

 kind of reasoning is already sufficiently proved. 

 I am, Sir, 



Your most obedient servant, 

 THOMAS KNIGHT. 

 PapcasilCf Nov. 3d, ISIO. 



VI. 



Description of an Hygrometer for GasseSt and the Method of 

 Using it, to subject different Substances to their Action : by 



Mr. GUYTON-MORVEAU*. 



Si 



'INCE philosophers have endeavoured to investigate the Ascertaining 

 properties of aeriform fluids, various kinds of apparatus *''®, "7"*^^* *^ 

 have been invented, for placing them in contact with sub- portant. 

 stances, that by their action on them might elucidate thei^ 

 nature, or might form with them new combinations. Of 

 latie they have particularly felt the necessity of reducing 

 gasses to the greatest dryness, that the phenomena produced 

 by their essential constituent parts might not be confounded 

 with those, that might result from the decomposition of the 

 water given out by them. 



The instrument, which I now lay before the class, appears ^hg innru- 

 to me well adapted to both these objects. It was not till i"^'^' repeat- 

 I had several times tried it, that I resolved to have one ^ ^ 

 finished with great care, and to communicate a description 



*■ Annal. de Chim. vul, LXV!!!, p. 5. The instruraant here described 

 was laid beCoie the physical and mathematical class of the Institute oa 

 the 8th ot August, 1808. 



of 



