VI. HYGROMETERS. 583 



2861. Psychrometer Scale, for determining the relative and 

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

 calculation. Model with explanation. Professor Prestel, Emden. 



2862. August-Fsychrometer, with two thermometers di- 

 vided into i- on a stand. Warmbrunn, Quilitz, Sf Co., Berlin' 



2863. Catgut Hygrometer, dating from the first quarter of 

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



Sub- Committee of Breslau (Professor 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, Kew Observatory. 



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



M. Henri de Saussure, Geneva. 



Original models having belonged to himself, accompanied by tables drawn 

 up by him and his son Theodore de Saussure. 



2867. Hair Hygrometer of De Saussure, with two gra- 

 duations, one being fractional of relative moisture. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



The faults found with the hair hygrometer are caused, generally, by the 

 very great imperfection of the manufacture of those usually sold by the trade. 

 The hair deteriorates, and from time to time its indications alter, because, as 

 a rule, the weight of tension is too great. Hair properly prepared, and subject 

 to due tension only, altered so little that M. Regnault tells of having found 

 an old hygrometer made by Paul as correct as any modern instrument with 

 which he has compared it. Another cause of irregularity proceeds from the 

 careless choice of the hair. All hair that has been pulled about, and of which 

 the limit of elasticity has been exceeded, should be avoided. The most 

 isolated hamlets have now to be searched in order to obtain hair uncombed. 

 The object of the Geneva Association for constructing Scientific Instruments 

 is to rehabilitate the hygrometer of De Saussure by an improved construction. 

 The general formation of the De Saussure hygrometer has been maintained as 

 being the best, but the following modifications have been introduced: 

 1st. The marking needles by the axle of the pulley are made of bronze 

 aluminium, thus making the pulley lighter, consequently more move- 

 able, and lessening the friction of the axle. 



2nd. The weight of tension of the hair is replaced by a gold spiral, which 

 makes the instrument more portable and avoids the twitching of the 

 hair by oscillation and accidental displacement of the weight. 

 3rd. The hygrometer has two graduations (this is the chief modification) ; 

 the first is an arbitrary division in equal parts of to 100 ; the second 

 marked out on a inoveable arc, is superposed to the first and registers 

 in hundredths the relative moisture, or the fraction of saturation. 

 Thus, when the hygrometer registers 50, it is certain that the air 



