466 



evaporation in still air, the maximum depression will not be attained. 

 To produce the maximum effect, the bulb and its envelope must be ex- 

 posed to a brisk current. When there is no natural current, an artificial 

 one must be created, such as by a bellows, a fanner, or by swinging the 

 bulb. Such a proceeding will reduce the mercury to the lowest degree 

 that is possible by the action of the air. Even in the open air, unless 

 the breeze be strong, artificial aid of this kind must be employed. As 

 already observed, there is often great difficulty in coming to a just con- 

 clusion, in consequence of the rapid changes which the air experiences 

 in temperature and hygrometric condition. It is to the case of the 

 open air that the difficulty applies ; for the experiment may be easily 

 and correctly made when an artificial current is created. But in every 

 case, even in a brisk breeze, it will be prudent to assist the evaporation 

 by the fanner, so that the utmost depression may be certainly obtained. 

 The kind of fanner is far from being immaterial ; for not only must 

 evaporation from the bulb take place, but the resulting aqueous vapour 

 must be removed from the immediate vicinity of the evaporating surface. 

 This will be best effected by the use of a large fanner. I found a strong 

 pasteboard, eight inches square, with a firm handle fixed to one side, 

 to answer best ; it should be used by quick extensive swinging strokes, 

 for such will generally sink the mercury half a degree lower than rapid 

 short strokes. If a bellows be used, the nozzle must be kept about fifteen 

 inches from the bulb ; if very near, the blast will cause a slight elevation 

 of the mercury, instead of a reduction, as was long ago observed by 

 Cassini and de la Hire (" Memoirs of the Eoyal Acad., Paris," 1710). 



I conceive that the want of agreement, noticed by Dr. Apjohn, be- 

 tween the observed and calculated dew-points, in some of his experiments 

 ("Trans, of the Eoyal Irish Academy," vol. xvii., p. 291) is not attribut- 

 able to the coefficient having been assumed too great, but that the depres- 

 sions were too small ; for he omitted to promote evaporation from the 

 wet bulb by artificial means ; yet such expedients are necessary to the 

 attainment of the correct elasticity of the aqueous vapour. 



Having thus described the two instruments which I have combined 

 into one, it remains to make some observations on the question whether 

 the Psychrometer is capable of affording true results by the adaptation 

 of certain calculations to its indications. Many years since, Professor 

 Apjohn communicated to the Royal Irish Academy a formula for finding 

 the dew-point by the wet bulb Thermometer, and described many 

 experiments which showed that the calculated dew-points agreed in a 

 striking manner with those obtained by experiment. The following is 

 his formula : — 



/"=/,_^ + JL, 



in which f" is the tension of steam at the dew-point ; /', its tension at 

 the temperature of the wet bulb ; d, the depression, or difference between 



