28 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



It is important that the muslin covering for the wet bulb be kept 

 in good condition. The evaporation of the water from the muslin 

 always leaves in its meshes a small quantity of solid material, which 

 sooner or later somewhat stiffens the muslin so that it does not 

 readily take up water. On this account it is desirable to use as pure 

 water as possible, and also to renew the muslin from time to time. 

 New muslin should always be Avashed to remove sizing, etc., before 

 being used. A small rectangular piece wide enough to go about one 

 and one-third times around the bulb, and long enough to cover the 

 bulb and that part of the stem below the metal back, is cut out, 



FiGCUE 11. — Sling psychrometer (Bureau of Mines type). 



thoroughly wetted in clean water, and neatly fitted around the ther- 

 mometer. It is tied first around the bulb at the top, using a mod- 

 erately strong thread. A loop of thread to form a knot is next placed 

 around the bottom of the bulb, just where it begins to round off. 

 As this knot is drawn tighter and tighter the thread slips off the 

 rounded end of the bulb and neatly stretches the muslin covering 

 with it, at the same time securing the latter at the bottom. 



Certain precautions must be observed in preparing and fitting the 

 inuslin covering of the wet bulb as follows : 



(1) Fine, preferably loosely woven, muslin is the best, and, if new 

 material is used, it must first be thoroughly washed with water to 

 remove all sizing as far as possible. 



(2) The covering must be carried up on the stem of the thermom- 

 eter above the bulb a distance so that the stem near the bulb, as well 

 as the bulb itself, is cooled by the evaporation. 



