110 MR. W. N. SHAW ON HYGROMETRIC -METHODS. 



(Experiment 80). Certain of the experiments show that a dew deposit is formed at 

 a temperature above the theoretical dew-point. Thus in Experiment 82 the deposit 

 is obtained at 1'22, the theoretical dew-point being 0'80 ; and in Experiment 83 a 

 deposit was obtained at 16'22, the theoretical dew-point being 15'02. These read- 

 ings were obtained many times successively, so that they cannot be assigned to want 

 of uniformity in the temperature. They show that a visible deposit is obtainable 

 upon a glass surface when the air is not saturated, and the difference is larger than 

 in the case of the REGNAULT hygrometer, amounting to a whole degree in Experi- 

 ment 83. 



It, therefore, seems that the result obtained from observations with DINES'S hygro- 

 meter is likely to give a very easy determination of the dew-point, that is within 

 small limits of error ; but, that if it is observed with the closest attention, the result 

 will be considerably too high, in consequence of the premature formation of a dew 

 deposit, and it may be erroneous in consequence of the variations in temperature of 

 the different parts of the box containing the thermometer. This latter effect may be 

 almost entirely avoided by using for forming the deposit water which is itself of a 

 uniform temperature and very slightly below the dew-point, so that a large quantity 

 may be required to flow for a small change of temperature, or by the method sug- 

 gested by Professor CHRYSTAL, and in use by some of the Scottish observers, as by 

 Mr. H. N. DICKSON (see ' Edinburgh, Roy. Soc. Proc.,' vol. 13, pp. 199 and 951). 

 But the instrument can hardly be regarded as a standard one. 



The deposit upon the glass surface above the dew-point is, I think, well established ; 

 it was suspected by REGNATJLT (see p. 121), and appears to be less obvious in the case 

 of silver than of glass. The difference may, however, be due to the greater difficulty 

 of observing it. 



3. ALLUARD'S INSTRUMENT. 



25. A description of this modification of REGNAULT'S instrument is given in 

 Note A (p. 141). The difference between the two lies entirely in the facility of 

 reading. ALLUARD encloses his thermometer in a brass box with gilt and polished 

 sides, and surrounds one side with a brass plate so cut that the side of the box is 

 surrounded by a surface of brass in the same plane with it. It is easier and less 

 uncertain in reading than REGNAULT'S instrument. The best way of illuminating it 

 is similar to that suggested for DINES'S hygrometer, namely, to set up a small patch of 

 black paper in the window and arrange the reflecting surface of the hygrometer so 

 that the eye sees the reflection of the black paper. A very faint deposit of dew is at 

 once visible. I have not been able to arrange the instrument so that I could observe 

 its behaviour in air of a known state. The instrument is too bulky for me to do that 

 without apparatus on a much larger scale than mine. I cannot, however, think that 

 such a course is necessary. The observations cannot differ from those with REGNAULT'S 

 instrument, except in regard to the facility of seeing deposits. For that reason it is 



