INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 51 



I]i order to obtain tlic most nearly correct results, the muslin should 

 be thoroughly saturated, but not coated with a superfluous film of 

 water. If too much water surrounds the thermometer bulb, the in- 

 dicated wet-bulb temperature will be too high. Similarly, if the mus- 

 lin is only partially moistened, the reading will ao-ain be too high. 

 Trouble of either sort can usually be remedied by adjusting the wick. 

 The w^ater that is supplied to the muslin must at all times be pure 

 and free from dissolved substances, such as sea salt; otherwise, in- 

 correct wet-bulb temperatures will be indicated. To this end it is 

 necessary that the water supply be renewed and the wick washed at 

 frequent intervals. 



The results obtained through the use of stationary thermometers 

 are unsatisfactory, and to meet the needs of modern weather science 

 it is advisable to employ a more reliable type of apparatus. An 

 alternative and more accurate method of measuring wet-bulb tempera- 

 tures on shipboard is afforded through the use of the sling psy- 

 chrometer, a full description of which is given on pages 27 to 29. 



In making an observation with the psychrometer the first step is 

 to saturate the muslin-covered wet bulb with pure water. This is 

 most easily accomplished with the aid of an eye-dropper or small 

 syringe. When this operation has been completed, the observer 

 should take up his station at some well-exposed point on the weather 

 side of the ship and hold the psychrometer at arm's length, outside 

 the rail whenever possible. If the apparent wind at the point of 

 observation is force 4, or greater, the natural ventilation thereby 

 produced is ample and the instrument need not be whirled. It may 

 simply be held stationary and freely exposed to the wind. However, 

 when the apparent wind is less than force 4, artificial ventilation 

 is required. The thermometers are then wdiirled rapidly for at least 

 30 seconds ; stopped and quickly read, the wet bulb first. This read- 

 ing is kept in mind, the psychrometer immediately whirled again, 

 and a second reading taken. This is repeated three or four times, or 

 more if necessary, until at least tAvo successive readings of the wet 

 bulb are found to agree closely, thereby showing that it has reached 

 its lowest temperature. Two minutes or more is generally required 

 to secure the correct temperature. 



When the air temperature is at or below the freezing point, or even 

 slightly above it, it very often happens that the wet-bulb tempera- 

 ture will fall below the freezing point before the water actually 

 freezes. This phenomenon of supercooled water may always be rec- 

 ognized when it occurs, since the sinking mercury does not stop 

 at the freezing point but falls rapidly below it. No error results 

 on this account, provided the minimum temperature is reached. How- 

 ever, after a longer or shorter interval the water suddenly begins to 

 freeze, heat is liberated, and the temperature of the wet bulb in- 

 stantly rises to 32° F. (0° C). It will remain at this value until 

 all the water that covers the wet bulb has frozen. Subsequently it 

 will begin to fall again. In such cases it is necessary to continue 

 the ventilation until the ice-covered bulb has reached a minimum 

 temperature. 



Wliile the psychrometer will give quite accurate indications, even 

 in bright sunshine, yet observations so made are not without some 

 error. Therefore, it is desirable to hold or whirl the psychrometer 



