582 SEC. 14. METEOROLOGY. 



VI. HYGROMETERS. 



2853. Saussure's Hygrometer ; an old specimen by V. F. 

 Hausman. G. J. Symons. 



2854. Whalebone Hygrometer, by Thos. Jones, of Oxendon 

 Street. G. J. Symons. 



2855. Daniell's Hygrometer. No maker's name, but 

 formerly belonging to Sir James South. G. J. Symons. 



2856. Balance Hygrometer (drawing and essential part of 

 the instrument). Professor Buys- Ballot, Utrecht. 



At one arm of a balance is hung a wide glass tube filled with chloride of 

 calcium or any other hygroscopic substance. This tube is closed at its 



upper part by a cork stop, bearing two I M bent glass tubes plunging in oil 



baths, but care is taken -not to submerge their open ends. Two glass 

 bells, ending in tubes at their upper parts and plunging for nearly half an 

 inch in the same oil, render it possible to aspirate air through the chloride of 

 calcium tube without affecting its movability. One of the bells is joined by 

 an india-rubber tube with the spot the air of which is to be examined ; the 

 other with a gas meter and aspirator. In a given time you can in this 

 manner weigh the water contained in a quantity of air indicated by the gas 

 meter, and so conclude as to its humidity. It is obvious that the instrument 

 can easily be made self-registering. 



2857. A number of Hygrometers and Psychrometers. 



Dr. H. Geissler, Bonn. 



2858. Klinkerfues' Bifilar Hygrometer, with reduction 

 disc, executed by W. Lambrecht, Gottingen. 



Professor 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. 



The reduction discs for the psychrometer give likewise the dew point accord- 

 ing 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 yet a second division, which comes into use in the case of the evapora- 

 tion temperature falling below zero. 



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

 b, the quantity ^L (6 750), taken in nearest round numbers, can be easily 

 multiplied in the head by the thermometric r 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. 



2859. Reduction Discs for Psychrometers. 



Professor Klinkerfues, Gottingen. 



