of a New Hygrometer. 25 



the size of the orifice in the cup c which is oval, is f of an 

 inch in length by ^ in breadth. 



When not in use, a box or brass case covers the whole in- 

 strument, and screws on the stand M at tiw. The scale is 

 made of ivory and ought to be as slight as possible.* 



The principle which Dr. Hutton proposed I considered 

 the best : — 



1st. As by it the temperature of the shade could be ascer- 

 tained at the same time along with its hygrometric condition. 



2nd. Because it would indicate the relative dryness of 

 the atmosphere ; and, 



3rdly. That the Dew-point could be easily ascertained 

 from the data afforded by the instrument ; whence the abso- 

 lute quantity of aqueous vapour could also be estimated. 



I was led to the construction of my instrument by the 

 following observations in the Encyclopaedia Britannica, 

 which I accidently saw in the winter of 1834, during my 

 detention at Portsmouth, by the S. W. gales, when on my 

 way to Madeira; *'a more scientific instrument has been 

 constructed on the following principle, viz. : — that as eva- 

 poration produces cold, the effect of that process on a ther- 

 mometer will indicate the rate and amount of evaporation, 

 and consequently the relative moisture of the air. The 

 more dry the air is the greater will be the evaporation ; and 

 the greater also the cold or the depression of the mercury 

 in the thermometer." — "The general doctrine is that the 

 dryness of the air, under all circumstances is precisely indi- 

 cated by the depression of temperature produced on a humid 

 surface which has been freely exposed to its action." 



Not possessing either Professor Daniell's, or Sir J. Leslie's 

 Hygrometers, I was led by these remarks to the construction 

 of the instrument now proposed, being desirous of making 

 a minute series of observations during my residence in 

 Madeira, for the purpose of determining the true character 

 of its climate ; the ample details of which will be given in 



• The instrument is made in the above form by Mr. Gary, Optician, 181, Strand ; 

 and by Mr. Squire, Operative Chemist, 277, Oxford Street, London, to pack in a 

 neat travelling case, containing spare fountain, silk, &c., with appropriate tables 

 for the use of the instrument ; also by Mr. William McDowall, Philosophical In- 

 strument Maker, 13, Infirmary Street, Edinburgh. Much care is requisite in the 

 construction of the Hygrometer to ensure accuracy ; the above named makers may 

 be depended upon as aflFording accurate instruments. 



