AfK. W. N. SHAW ON HYGROMETRIC METHODS. 



Ill 



more suitable for use as a standard instrument, as the feeling of uncertainty, that is 

 very strong while working with REQNAULT'S, is much less with ALLUAED'S.* 



4. BOGEN'S INSTRUMENT. 



26. I need spend but few words over this instrument. It is a dew-point instru- 

 ment in which the cooling is produced by solution of ammonium nitrate contained in 

 a silvered glass vessel. The water is squeezed into the glass vessel by means of a 

 flexible india-rubber ball, and the temperature of deposition is read in the usual way. 

 It affords a fairly good lecture experiment, but continuous observations are quite 

 impracticable, as the apparatus has to be washed out as soon as one observation is 

 taken, and the delay and trouble are too great. There is, besides, no possibility of 

 graduating the fall of temperature. I have made a few observations with it, but it is 

 not worth while to recall them. 



* On turning back to some results that I obtained in 1883, I find considerable differences in the 

 readings of different dew-point instruments. I was at that time comparing all the different instruments 

 that I had, viz., ALLUARD, RKONAITLT, DINES, hair-hygrometer, and a large number of wet and dry bnlbs, 

 whose indications were reduced by two sets of tables, viz., JELINEK'S and GLAISHEB'R. They were honest 

 attempts to obtain a comparison of the indications of different instruments, but the results were such 

 that they showed that a very systematic investigation was required before one could hope to obtain 

 concordant observations. I quote a table of results here. 1 made the assumption that by driving the 

 air of the room over all the instruments by means of a rotary fan I should obtain all ttie instruments 

 exposed to air in the same state. I fear that could not have been correct. I used the chemical method 

 * a check, but I have since that time seen means of improvement, and I only offer the table as a 

 specimen of the difficulties of a hygrometrio observer. 



TABLE of Vapour Pressure in Room, as deduced from Observations with various Instruments, 1883. 



I had previously attempted to compare all the dew-point instruments together in the manner described 

 in my preliminary report of May 10, 1881, but after a few observations I -came to the conclusion that any 

 such attempt was worthless unless a circulation of air was provided, observations of different hygrometers 

 in a closed vessel not giving comparable results. 



