504 



SCIENCE. 



[Vol. I., No. 18. 



evident that the ice had entirely evaporated. 

 At the 5.29 reading, Regnault's formula gives 

 a dew-point of 25.3°, and the condensing hj'- 

 grometer gave at the same time a dew-point 

 20.4°. In the second set at 6.54, seventy-seven 

 minutes after wetting, the dew-points were 24.0° 

 and 19.7° respectively. It will be seen that the 

 length of time required in the last set (seventy- 

 seven minutes) is entirely too great for good 

 results, as in this time the temperature may 

 change several degrees ; and there is so great 

 uncertaintj' in the length of time required, that, 

 to obtain a good result, it would be essential to 

 wet the bulb an hour and a quarter or an hour 

 and a half before the time, and then note the 

 temperature from time to time in order to 

 catch it when it has ceased falling. The above 

 conditions of observation are ordinarily im- 

 practicable, and, besides, the final results, 

 showing dew-points about 4.5° higher than the 

 condensing hygrometer, are entirely unsatis- 

 factory-. 



I have investigated' the effect of an induced 

 air-current as a means of effectually removing 

 these and other objections. Experiments were 

 tried with fans, common hand-bellows, and a 

 Casella whirling apparatus. All of these trials 

 showed, that, with a velocity' of the air-current 

 ranging from 1.5 to 5 metres per second, the 

 readings of the dry- and wet-bulb hygrometer 

 are nearly identical. 



The length of time required to bring down 

 the wet-bulb temperature rarely exceeds two 

 minutes : in only one extreme case did it I'c- 

 quire thirteen minutes. If it be objected that 

 anj' form of motor for producing an air-cur- 

 rent must necessaril}' compress the air, and by 

 heating it vitiate the results, it may be said 

 that the compression need be very slight. 

 Experiment shows that the induced current 

 produces, if au}^ thing, a lower temperature, 

 at least in the winter season ; and, since the 

 air-current reaches both thermometers, the dif- 

 ferential results will not be affected. 



The most satisfactory showing of experi- 

 ments with an induced air-current, however, is 

 that uniform and accurate results ma}^ always 

 be obtained at temperatures as low as 10° 

 (which is the limit that has occurred the past 

 winter) , as determined by comparison with a 

 Regnault's condensing hygrometer; and un- 

 doubtedl3' the same would be found at tempera- 

 tures even below 0° F. The simplest motor 

 for the induced current for an}^ exposure, ex- 

 cept from a window, is a common fan ; another 

 convenient form, and one by far the easier to 

 use, is the hand-bellows. For a window-shel- 

 ter, the latter can be readily rigged with a pulley 



and string so as to be operated from within ; 

 and this is the form used by myself. I have 

 mentioned above the whirling apparatus of 

 Casella. This, though giving good results, is 

 much more complicated and expensive, and 

 is, moreover, unsuited to a window-shelter. 

 There are manifold other forms of motors, but 

 it is doubtful if they would be anj' better than 

 those already described. 



I have carefully measured the induced air- 

 currents with a Casella air-meter, and have 

 found that a fan making a hundred strokes 

 a minute in one direction, and placed within 

 three or four inches of the meter, gives a 

 velocity of 1.5 metres per second ; that a bel- 

 lows of a litre capacity, making fifty strokes to 

 the minute, at a distance of six inches, gave a 

 velocity of 2 metres per second, while at twelve 

 inches it gave 1.8 metres per second ; and that 

 the whirling apparatus easiU' revolved the ther- 

 mometers at the rate of 5 metres per second. 



The expense of a fan would be nominal ; 

 a strong hand-bellows, with all necessary ap- 

 pliances, ought not to cost more than $2.50. 



In order to exhibit the advantage • to be 

 gained by ventilating this hj-grometer, I give 

 the ■ following table, containing observations 

 with it, and, for comparison, those with the 

 condensing hygrometer, as made at seven 

 A.M. during twenty-nine daj's of March, 1883. 



From this table we see that columns 6 and 

 7, which contain dew-points computed from the 

 ventilated hj-grometer, and determined bj^' Reg- 

 nault's condensing hygrometer, respectivelj', 

 show a close agreement ; the difference of 2.1° 

 between the means being due in part to the 

 formula of reduction used with the dr^-- and 

 wet-bulb instrument. 



Columns 8 and ,9 show a mean monthly differ- 

 ence in the relative humiditj-, by the unventi- 

 lated and ventilated bulbs, of 10%, and an 

 extreme difference of 26%, for a single obser- 

 vation, in favor of the ventilated. 



Since conducting the above investigation, 

 mjr attention has been called to similar work 

 done bj' Mr. Sworykin in Russia. The means 

 of fifteen observations, as given by him, are 

 as follows: mean air-temperature, 21.5° F. ; 

 relative humidity, unventilated 59%, ventilated 

 55% ; mean velocitj' of wind during the ob- 

 servations, 11 miles per hour. 



The formulae of reduction used in this paper 

 are those determined bj' Regnault. He himself 

 declared these unsatisfactory ; but they are the 

 best we have, and certainlj', as m3' experiments 

 have shown, very superior to the factors of 

 Glaisher. Manj' verj' carefully conducted ob- 

 servations at temperatures below 0° F., and at 



