APPENDIX. 275 



always 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. 

 This will be the case if the muslin does not readily become wet after being 

 dipped in water. On this account it is desirable to use as pure water as pos- 

 sible, and also to renew the muslin from time to time. New muslin should 

 always be washed to remove the sizing, etc., before used. A small rectangu- 

 lar 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, thoroughly wetted in clean water, and neatly fitted around 

 the thermometer. It is tied lirst 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. 



To make an observation — The so-called wet bulb is thoroughly saturated 

 with water by dipping it into a small cup or wide-mouthed bottle. The ther- 

 mometers are then whirled rapidly for fifteen or twenty seconds, stopped and 

 quickly read. A mental note of the readings is made, when they are again 

 whirled and read. This will be continued until the wet-bulb thermometer 

 ceases to fall, when the readings of the two thermometers should be read 

 and recorded. If the wet thermometer should read '.il°, the whirlings and 

 frequent I'eadings should be continued for a considemible time, to be certain 

 that a further fall will not take place. The freezing of the water at 32° 

 causes the fall to be checked for a short time. 



Subtract the reading of the wet thermometer from that of the dry. Find 

 this difference in the line at the top of the table. The dew-point will be 

 found at the intersection of the column beneath with the line which has the 

 l)roper dry thermometer reading at the left. 



FIRST EXAMPLK. 



Dry-bulb thernioiiieter 55° 



Wet^bulb thermometer 44 



Difference 11 



Dew-point from table --- 30 



SECOND EXAMPLE. 



Dry -bulb thermometer 43° 



Wet-bulb thermometer 38 



Difference 5 



Dew-point from table 31 



