60 Analyses of Books, [July, 



" The attention of the scientific world has been lately so much 

 occupied in experiments on atmospheric phaenomena, that it is 

 hoped any simphfication or improvement in tlie instruments 

 employed for that purpose may not be unacceptable. 



" The principle of the hygrometer which I am about to 

 describe, is that of enabling the observer, readily and accurately, 

 to ascertain by direct and simple means, the degree of tempera- 

 ture at which the moisture of the atmosphere is condensed, and 

 the instant at which that operation commences. 



" The hygrometer is composed of a mercurial thermometer, the 

 graduated scale of which is about four inches and a half long ; 

 at the lower part of the scale the glass tube is bent to form a 

 right angle, at the end of which the bulb of the thermometer 

 rises parallel to the scale, and about one inch from it ; the bulb 

 is about one inch long, and of a cylindrical form, with a black 

 convex top, the diameter of which is a little more than that of 

 the cylindrical part, which is covered with silk. The scale is 

 attached to a piece of cylindrical wire, three inches long, and 

 turns upon a joint screw passing into its edge, the other end of 

 which wire being placed in a tubular foot fixed to the inside of 

 one end of the case, forms a stand for the instrument. The case 

 contains a small bottle for ether. 



" The thermometer thus constructed will give both the tem- 

 perature of the air and that of the dew-point, which last is 

 effected by placing the mouth of the bottle containing the ether, 

 in contact with the upper part of the covered surface of the bulb, 

 when, by gently inclining the bottle, the ether will flow down- 

 wards without wetting the top of the bulb, which will almost 

 immediately become dull by the deposition of moisture on its 

 surface ; when the observed temperature may be taken and the 

 difference ascertained. 



" Should it be objected against the principle of the instrument 

 here proposed, that the indications do not exhibit the true tem- 

 perature of the upper surface of the bulb, on which the deposi- 

 tion of dew takes place, but that of the lower part, to which the 

 ether is applied ; it may be answered that by inclining the 

 whole instrument so as to render the axis of the bulb horizontal, 

 and establish thereby a free circulation of the mercury in every 

 part, this objection may be obviated ; but on repeated trials I 

 have not found this to produce any diflerence in the results. 



" I ought also perhaps to mention that an instrument somewhat 

 similar in principle has been used in Vienna, and was mentioned 

 by Prof. Baumgarten of that capital to a friend, who communi- 

 cated the fact to myself." 



A representation of this hygrometer is given in an accompany- 

 ing plate. E. W. B. 



{To be continued,) 



