On an Instrument for ascertaining the Dew Point. 61 



it can be satisfactorily proved that such works can be made 

 reproductive, and handed down to posterity as the triumph of 

 the infant age of Victoria, and worthy of the times in which 

 we live. 



Art. IX. — On the Construction of an Instrument for ascer- 

 taining the Dew Point. By B. BROUGH Smyth, Esq., 

 C.E., F.G.S., Sfc. 



[Read before the Institute, 6th May, 1857.] 



Great difficulty is experienced by Meteorologists in deter- 

 mining the dew point by direct experiment. 



In very hot countries, or in those places where the air is 

 very dry, Daniell's beautiful invention is almost valueless. I 

 need not state the objections to the black and white bulbs of 

 Daniell: they are known to all who have ever used a 

 hygrometer systematically, and compared the results with 

 the dry and wet thermometers. 



Mr. Glaisher has emphatically protested against the use of 

 Daniell's instrument in hot countries, and indeed has very 

 properly pointed out the liability to error when it is used, 

 under any circumstances, by inexperienced persons. Even 

 with the utmost care the best result is seldom within 0-25°. 



An ordinary method of obtaining the temperature of the 

 dew point is by a silver cup, and a freezing mixture. The 

 cup is partly filled with water, and is cooled down by stirring 

 in the refrigerating compound until a deposit of clew takes 

 place on the outer surface of the cup, and at the moment 

 when the dew is observed the temperature of the liquid is 

 taken by a thermometer. 



Now all I have to offer as a contribution to the instru- 

 mental aids of the Meteorologist is, an improvement on this 

 last method. 



The drawing shows at a glance the plan I would pi'opose. 

 The bulb of the standard Kew thermometer A is placed close 

 to the inner edge of the thin gold cup B. Within the gold 

 cup there is a copper vessel, C, connected by a (Y) pipe 

 with the exterior cups C, E, and F. These are filled with 

 water, and the temperature of E is supposed to be reduced 

 to 33 ° , or lower when it may be required, by a freezing 

 mixture. By turning the stopcocks, x x, the observer can 

 cause the gold cup to be filled with water at any required 



