and the Psychrometer. 437 



ditions of the air, certainly we can depend upon the indications 

 of each by itself at all times. It is the object of this paper to 

 indicate a practical method of using these instruments, to con- 

 tribute toward the establishment of a law controlling them, 

 and to show what accuracy may be attained by either of them. 

 The importance of a practical discussion of this question will 

 be readily recognized. In many instances weeks and even 

 months have been spent in an endeavor to obtain satisfactory 

 comparisons. A most significant sign of the present state of 

 the problem is to be found in the fact that the International 

 Meteorological Committee which has in charge the duty of com- 

 puting meteorological tables have abandoned the project of pre- 

 paring tables for the psychrometer. When we add to this the 

 fact that large sums have been expended in preparing and 

 printing the existing tables, which are so discordant among 

 themselves, the necessity of a further investigation becomes 

 very apparent. 



The Condensing Hygrometer. 



One instrument of this class, that of Mr. Dines, is very sim- 

 ple in construction and not very expensive. In this, cold water 

 or melting ice is employed as a cooling agent, the dew being 

 deposited on a plate of black glass, underneath which the cool- 

 ing liquid passes, the temperature being indicated by a ther- 

 mometer near the plate. There are serious objections to this 

 instrument for accurate work. The presence of so much water 

 in the neighborhood of the plate vitiates the result; with a 

 properly arranged Alluard apparatus, shortly to be described, 

 the harmful effects from an open bottle of water on the same 

 table may be easily recognized. Moreover, this instrument 

 cannot be used at temperatures of the dew point below 85°. 

 There is little doubt but that a properly ventilated psychro- 

 meter will give much more accurate results for moisture con- 

 tents of the air than can be hoped for with this instrument. 

 Another instrument of this class is the Alluard form of Beg- 

 nault's hygrometer. In this the usual cylindrical silver cup 

 gives place to a gilded prismatic box. The usual tubes, one 

 running to the bottom and the other from the top, are supple- 

 mented by a third running to the top and terminating on the 

 outside in a little funnel by which the box may be conveniently 

 filled with the cooling fluid. Of the latter, both ether and 

 rhigolene were tried aud preference given to the rhigolene. 

 This may be evaporated more readily, the residuum left after 

 evaporating a large quantity is by no means as great or as 

 harmful in stopping farther evaporation as is the case with 

 ether; finally, it has no affinity for moisture. This latter point 

 is important ; in one instance, an operator with ether continu- 



