June 8, 1883.] 



SCIENCE. 



503 



book just issued, ' the diy- and wet-bulb hy- 

 grometer fails.' Some of the directions are 

 as follows : — ■ 



Wet the muslin with a camel's-hair brush or 

 a sponge fifteen minutes before the observa- 

 tion. The film of ice should be as thin as possi- 

 ble. Remove the muslin, and wet with brush. 

 Wet, by raising a cup and immersing the bulb 

 for a moment, twenty minutes befoi'e the read- 

 ing. Wet some time, say an hour, before the 

 observation. Wet immediately after a reading, 

 and it will be readj- for the next. One author- 

 itj' suggests, that, if the air is still, it is well 

 to increase the evaporation by a fan. Reg- 

 nault has established that no appreciable error 

 is introduced bj- an air-current as high as five 

 or six metres per second (metres per second 

 may be readily converted into miles per hour bj' 

 using the factor 2.24). The Italian govern- 

 ment, some 3'ears ago, introduced an induced 

 air-current in their hygrometric observations. 



Relative humidity at 7 A.M., February, 1883. 



Deduced from observations of the dry- and wet-bulb hygrometer 



As an illustration of the varying results ob- 

 tained bj' the common method of observing 

 this hj-grometer, I have given the preceding 

 table, showing the relative humiditj- at four 

 stations in Washington. 1. Kendall Green. 

 This station is situated about a mile and a half 



north-east of the capitol, and has an exposure 

 of thermometers some fifty feet above ground. 

 2. Fort Mj^er, situated about three miles west, 

 and has an exposure about forty feet above 

 ground. 3. West Washington, situated about 

 three miles west, with an exposure about 

 thirtj- feet above ground. 4. Naval observa- 

 tory, about two miles west, with an exposure 

 four feet above ground. 



This table shows an extreme difference of 

 35% for a single observation. The very high 

 per cent found at the observatorj- is due in part 

 to the exposure being so near the ground. 

 This suggests an interesting subject for inves- 

 tigation. It has been determined b}' experi- 

 ment in Europe, that, with proper precautions, 

 the actual air-temperature is the same, whether 

 measured at five or a hundred feet above 

 ground. Now, if it be found that the lower 

 exposure gives higher percentage of moisture, 

 due to the settling of fog-banks or strata of 

 damp air, it becomes a matter of the highest 

 importance to ascertain the differences in 

 moisture in different strata, and to settle upon 

 some uniform height for all hygrometric obser- 

 vations. 



During the past winter, I have made a large 

 number of readings, hoping to remove some of 

 the recognized difficulties in this class of ob- 

 servations. The exposure of the hj-grometers 

 was from a north window fortj' feet above 

 ground. Great care was taken to exclude all 

 heated currents. The temperatures were from 

 10° to 50° F. 



As an example of these observations, I ap- 

 pend a table exhibiting two sets of readings 

 taken on Feb. 13, 1883. The air was perfectlj^ 

 still, and the pressure was 30.40". The read- 

 ings were made at intervals, as shown in the 

 table, without disturbing the instrument. 



Readings of dry- and -wet-bulb hygrometer on 

 Feb. 13, 1883. 



As the temperature of the wet bulb was 

 rising at the last observation in each case, it is 



