VI. HYGROMETERS. 701 



2858. Klinker fues' Bifilar Hygrometer, with reduction 

 disc, executed by W. Lambrecht, Gottingen. 



Prof. Klinkerfues, Gottingen. 



The bifilar hygrometer shows the relative dampness without further reduc- 

 tion, upon a stereotyped scale of equal divisions, and also the dew point by 

 means of the reduction disc. 



2859. Reduction Discs for Fsychrometers. 



Prof. Klinkerfues, Gottingen. 



The reduction discs for the psychrometer give the dew point according to 

 the following rule : 



The outer disc is turned round the inner one in such a manner that the 

 two places of the evaporation temperatures, read off from the moist thermo- 

 meter, coincide ; with the place of the air temperatures upon the one will then 

 also coincide the place of the dew point temperature upon the other. The one 

 disc has a second division, which comes into use in the case of the evaporation 

 temperature falling below zero. 



The barometric pressure is assumed to be 750 mm. ; for any other pressure, 

 b, the quantity -^ (b 750), taken in nearest round numbers, can be easily 

 multiplied in the head by the thermometric difference, likewise taken in round 

 numbers. The product expresses the number of hundredths of a degree, and 

 has to be added to the air temperature, in order to obtain, after the setting of 

 the disc, the dew point with greater precision. This correction is, however, 

 seldom required in practice. 



2861. Psychrometer Scale, for determining the relative and 

 absolute moisture of the air, as well as the dew-point, without 

 calculation. Model with explanation. Prof. Prcstel, Emden. 



2862. August's Psychrometer, with two thermometers 

 divided into 1 on a stand. Warmbrunn, Quilitz, $ Co., Berlin. 



2863. Catgut Hygrometer, dating from the first quarter of 

 the 18th century. Property of His Highness the Prince of Pless. 



Breslau Sub- Committee (Prof. PolecK). 



Interesting on account of its age. 



2864. Eight-Haired Saussure's Hygrometer, by Richer 

 of Paris, formerly the property of Mr. Francis Ronalds, and used 

 by him at the Kew Observatory in 1843. 



Kew Committee of the Royal Society. 



2865. Three Hair Hygrometers, by H. B. de Saussure. 



M. Henri de Saussure, Geneva. 



Original models, originally the property of M. H. B. de Saussure, accom- 

 panied by tables drawn up by him and his son Theodore de Saussure. 



2867. Hair Hygrometer of De Saussure, with two sets of 

 graduations, one being fractional of relative moisture. 



Geneva Association for the Construction of Scientific In- 

 struments. 



