702 SEC. 14. METEOROLOGY. 



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 the Construction of Scientific Instru- 

 ments is to revive the hygrometer of De Saussure in an improved form. The 

 general construction of the De Saussure hygrometer has been maintained as 

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

 1st. The marking needles on the axis of the pulley are made of aluminium 

 bronze, thus making the pulley lighter, consequently more moveable, 

 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 moveable 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 

 contains half the quantity of water that it can contain in the state ol 

 saturation. 

 The hygrometer is graduated according to " Regnault's " method. 



2868. Hygrometer, modified by Dr. Geissler, with a delicate 

 thermometer on a stand. 



Will. JTaak, Neuhaus am Renniveg, Thuringen. 



2869. August's Fsychrometer, on stand. The thermometer 

 divided into tenths of a degree from 30 to +45 C. 



Will. Haak, Neuhaus am Rennweg, Thuringen. 



2870. Psychrometer, August system, with stand, in a case. 



Ch. F. Geissler $ Son, Berlin. 



287Oa. Psychrometer, August system, with stand, in a case, 

 for travelling. Ch. F. Geissler 3? Son, Berlin. 



2871. Early Hygrometer. 



The Council of King's College, London. 



287 la. Oatbeard Hygrometer. G. J. Symons. 



287 Ib. Old Travelling Hygrometer. G. J. Symons. 



287 Ic. Modern Saussure's Hygrometer. G. J. Symons. 



2 87 Id. Mason's Wet and Dry Bulb Hygrometer for 

 Observatories. L. Casella. 



