44S H. A. Uazen — Condensing Hygrometer 



the first of these by 600 and the second by 750, the ap- 

 proximate air pressure at the two bights, we obtain -625 and 

 •632 respectively, showing that Ah was nearly constant and 

 that the relations to each other of the other three quantities are 

 practically the same at all altitudes. These observations, how- 

 ever, were not sufficiently numerous and were not made with 

 a psychrometer well enough ventilated to give satisfactory 

 results. 



In order to test the question a week was spent, in March, 

 1885, at Oakland, Md. s at a hight of 2,800 feet above sea level 

 and where the pressure was about 27". A series of compari- 

 sons at this point made with the identical instruments and under 

 precisely the same conditions as those previously made at sea 

 level* gave a value for Ah of "0197, while the sea level value was 

 •0204. If we assume h as 30" in both these cases, we shall have 

 A -000657 and -000680. The temperatures employed at Oakland 

 ranged from 102° to 14°, and with the former the dewpoint was 

 48° lower than the air. These observations were not sufficiently, 

 extended to give an absolute value for Ah, but the relative 

 comparisons may be regarded as approximately accurate. Any 

 one acquainted with the use of these instruments will recognize 

 the very small margin there is in the two values above. The 

 difference in the computed humidity from either of the values 

 will be nearly inappreciable. Moreover, whatever may be the 

 absolute relation between the dp : t and I' under different pres- 

 sures, none but the most refined observations can by any possi- 

 bility detect it, and so far as ordinary observations are con- 

 cerned no account need be taken of this difference up to 3,000 

 feet. This is especially apparent when we consider the extreme 

 difficulty under the most satisfactory conditions of obtaining 

 even an approximate value of the wet and dry temperatures, 

 and it should be borne in mind that these difficulties are not 

 due, for the most part, to accidental errors which may be aver- 

 aged out by multiplying the observations, but are caused by 

 constant errors, such as imperfect ventilation, too high temper- 

 ature through the day, and too low just after sunrise for an hour 

 or two, etc. The question of applying a correction for eleva- 

 tion to the indications of a psychrometer depends largely upon 

 the practical benefit to be derived. No such correction is 

 needed in order to make intercomparisons at high stations, but it 

 may be argued that it is needed in order to obtain, first, a com- 

 parison between a high station and a neighboring lower one, 

 and second, to obtain the diminution in moisture contents at any 

 time or during any season between sea level and the upper 

 atmosphere that is passing ;j high station. To these arguments 

 it may be answered, first, we do not know the variations of 

 humidity in different strata so as to obtain a law of reduction 



