106 



Ml!. W. N. SFIAW ON HYGROMETBIC METHODS. 



No. 81. 



The dew-point is put down as the temperature of appearance of dew in the last two 

 readings, as the deposit was extremely transient. It would appear from this that 

 dew may, with very great care, be seen at a temperature from 0'16 C. to 0'32 C. above 

 the dew-point. The other observations are merely ordinary dew-point determinations, 

 and may be affected by too high a reading being set down for the temperature of dis- 

 appearance ; but I had no doubt in the case quoted above. I find also the following 

 note in ray note-book for Experiment 82, when the saturator read 1'18 C. (corrected) : 

 " There is a change in appearance of the silver at 2'l (unconnected), and the dew 

 deposit is very slight even when the temperature is considerably below the dew-point. 

 The dew-point as ' naturally ' determined is given in the table. The plan of reading 

 deposit both on and off is quite useless, as the dew often disappears when the thermo- 

 meter shows a temperature below that at which dew was previously deposited." If 

 the change in appearance at 2'l indicated a deposit, allowing for thermometer correc- 

 tions, the deposit would have been seen at 1'88, when the theoretical dew-point was 

 1'18, in other words at a temperature of 07 above the saturation temperature. 



21. It would appear, therefore, that the third assumption of p. 101, viz., that 

 there is no deposit unless the temperature of the thimble is below the theoretical 

 dew-point, is not strictly true if the very first indications of a dew deposit are taken 

 into account. 



With the exceptions thus mentioned, the theoretical and observed dew-points are 

 in very close agreement. It may be said that at first an observer would probably 

 obtain too low a reading with REGNAULT'S instrument, and that with a fair amount 

 of practice his observed dew-points would probably agree on the average with the 

 temperature of saturation, although there might be exceptional observations with 

 somewhat widely diverging readings, depending possibly upon the illumination of the 



