SCOTT. — METEOKOLOGICAL INSTRUCTIONS. 17 



Edition) for each observation is to be compared with the reading 

 of the hair hygrometer taken at the same time which should be 

 entered in the " Remarks" space. The mean of these observations 

 for a few weeks when the temperature is above 32" is to be taken, 

 and thereby the correction for the readings of the hair hygro- 

 meter, from time to time, is to be ascertained. 

 The following is an instance : — 



Mean degree of Humidity by Wet Mean Reading of Hair 

 and Dry "Bulb Hygrometer. Hygrometer. 



^73, 77. 



correction to all readings of the hair hygrometer is accordingly 

 -4. 



Every possible opportunity must be taken at first to determine 

 these corrections for the hair hygrometer, inasmuch as it is not 

 to be expected that during the severe weather many such oppor- 

 tunities will occur. Whenever the Hair Hygrometer is readjusted, 

 a note to that effect should be inserted in the register. It will be 

 seen from the above that the object of reading the two different 

 classes of instruments above 32°, is to determine a correction for 

 the hair hygrometer while the wet and dry bulb themometer is 

 acting properly. For readings below 32°, the hair hygrometer 

 may be supposed to be a more trustworthy instrument than the 

 wet and dry bulb hygrometer, but both instruments should be read 

 together, and their indications entered in the register, as thereby 

 very valuable experience of the comparative utility of the two 

 instruments in the Arctic regions may be gained. 



In addition a pair of mercurial thermometers are to be mounted as 

 a dry and wet bulb hygrometer. The greatest care will be requisite 

 to ensure that these observations are correct when the tempera- 

 ture is below 32°. Inasmuch as it is to be expected that for a 

 long period the moisture on the wet bulb will be frozen, it will be 

 necessary to see that the muslin covering of the bulb is always 

 provided with a very thin coating of ice. This is effected by 

 moistening it with pure water at least once a day, and not less 

 than half an hour before an observation. The moisture on the 

 bulb will freeze at once, and the film of ice so formed will 

 in most cases last for 24 hours, inasmuch as at the very low 

 temperatures which will probably prevail the evaporation from ice 

 is very slight. 



Care should be taken that the water employed is always fresh 

 water ; no addition of spirit or of salt, in order to lower its freezing 

 point, is on any account to be admitted. 



In case of severe cold setting in, i.e., of temperatures below 

 — 20° F., it will be best to take in the mercurial thermometers and 

 make all the observations with spirit thermometers, but it should 

 never be forgotten that spirit thermometers are very deceptive, 

 unless great care is taken to keep them in order. They should 

 not be used if the spirit column be broken in any part by bubbles 

 or shortened by condensation of spirit at the top of the tube. 



Wind. — In addition to the ordinary observations of the estimated 

 direction and force of the wind, an electrical anemometer is 



