HYGROMETRIC RESEARCHES. 531 



execute the simultaneous ones at Ben Nevis. Being ordered to pass the winter in the 

 south, a series of determinations were made at the Montpellier meteorological station 

 through the kindness of Messrs Crova, Foex, Hondaille, and Legatur, who placed all 

 the resources of the meteorological, physical, and chemical departments at the writer's 

 disposal. Work at Fort- William was continued in the summer of 1894. Mr Mark 

 again went to Ben Nevis for hy geometrical work at Christmas 1893, and in the summer 

 and early autumn of 1894. Special simultaneous observations of dry and wet bulb 

 thermometers in the ordinary Stevenson screen and in Assmann's aspiration-psychrometer 

 were made by Mr Drysdale at Fort-William in August 1896, and at the mid-station 

 half-way up Ben Nevis in July and August 1897. 



Numerous notes have been received from Mr Omond and Mr Rankin at different 

 times. To all those, to the Ben Nevis Directors, and to Messrs James Wood and G. 

 Guyou, who have at various times helped with calculations, the writer would return his 

 grateful acknowledgments The kindness of the late Professor Tait, of Dr Buchan, and 

 of Mr Omond, requires special mention and thanks. 



Dry and Wet Bulb Thermometers and Vapour Pressures. 



The common method of ascertaining the humidity of the air is to observe the dry 

 and wet bulb thermometers placed in some screen, and from the readings obtained, to 

 calculate the pressure of water vapour present in the air, and the relative humidity, 

 which is the relationship between the actual vapour pressure and the maximum possible 

 at the temperature of the observation. The tables usually employed in this country 

 are those compiled by Glaisher from a series of comparisons between readings of 

 dry and wet bulb thermometers and simultaneous observations of Daniell's condensa- 

 tion-hygrometer. The classic researches of Regnault, on the pressure of saturated water 

 vapour at various temperatures, are utilised, the temperature at which dew appears on 

 the cooled, polished surface of the condensation-hygrometer being assumed to be that 

 at which the air would become saturated, were the mass of vapour to remain unchanged 

 but the temperature of the air to fall to that temperature. Hence a relationship 

 is found between readings of the dry and wet bulb thermometers and vapour pressure, 

 and the relative humidity can be calculated. 



Glaisher tabulates his ' factors,' which must be employed to multiply the difference 

 between dry and wet bulb readings so that the product may give the departure of the 

 dew-point from the actual shade temperature, and then uses Regnault's results. 

 Regnault, however, following August and others, expresses the dry and wet bulb 

 readings directly in terms of vapour pressure. The equation he employs is : — 



0-480 (t-f) Q-480 (t-t) 

 t = j — — ptttt j> — V when t is over 0. and t = t ^57: — T , — ' p when t is under C 



610 — £ r ■ J 689 — /- r 



where f is actual vapour pressure, f is the pressure of saturated vapour at the 



