MH. W. N. SHAW ON HYGROMETRIC METHODS. 107 



thimble. Finally, if very closely observed, the dew-point would be some tenths of a 

 degree too high, in consequence of a slight deposit formed above the true dew-point. 



DINES'S HYGROMETER. 



22. The method of introducing the instrument into the air to be investigated has 

 been already described, 8. The presence of the instrument caused at first consider- 

 able interference with the comparison of the saturator and chemical method. In 

 order to cool the glass surface upon which the deposit is formed, water, cooled by 

 dissolving ice in it, is made to pass along a metal tube (fig. 4), which runs close to 

 the stem of the thermometer t, and ends in the metallic box B, into which the ther- 

 mometer dips. The small cistern which holds the cooled water is in the ordinary 

 course screwed on to the leading tube at A. I found, however, that the neighbour- 

 hood of this cold reservoir caused a deposit of moisture in the tubes of the apparatus 

 near it, and in consequence spoiled Experiment 71, and the previous Experiments 68, 

 69, and 70 had been rendered useless by the deposit formed in the long metal tube, 

 which must, of course, be cooled below the dew-point and cause condensation before 

 the glass surface can be'cooled. So that the observations of dew-point with DINES'S 

 hygrometer reduced from these experiments are not of much value. In order to 

 avoid these defects the flow of water in the DIN EH was reversed, so that it had only 

 to pass through a very short length of tube before reaching the box, and the cistern 

 was separated from the rest of the apparatus by a considerable length of india-rubber 

 tubing. Further, the observations were only taken with DINES'S instrument at the 

 close of an experiment, after the drying tubes had been detached from the aspirator. 

 The apparatus was then connected with an aspirating air pump, and air drawn 

 through the saturator and the vessel containing the hygrometer. 



23. The observation of a dew deposit, upon the surface of the blackened glass is 

 a very easy matter, and much more satisfactory than with the REGNAULT thimble. 

 The arrangement that I have found to give the easiest observation is to attach a 

 piece of black paper to the middle of the window of the room, and look at the glass 

 surface from such a direction that it reflects only the black paper when there is no 

 dew on the surface. Under these circumstances a deposit shows itself with surprising 

 facility. The observations taken with DINES'S instrument were as follows : 



After Experiment 73. Saturator reading 20 0- 05 



DINES, 68 F. = 20'00 



0'05 diff. 



77. Saturator reading, 7'49. 



DINES first showed dew at 10'6, but no more at 8'3 ; on a 

 second trial dew at 7'8. 

 P 2 



