INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 27 



The sling "psychro meter. — There is another and well-known type 

 of portable ventilated instrument called the sling psychrometer, 

 with which it is possible to measure both air temperature and 

 humidity at sea with a reasonable degree of accuracy, at least with 

 greater accuracy than can be secured through the use of stationary 

 thermometers in a shelter. The sling psychrometer has the disad- 

 vantage that the two thermometer bulbs are not afforded protection 

 from rain and direct sunshine, but errors arising from the latter 

 source are, to a large extent, overcome by the strong ventilation to 

 which the aj^paratus may be subjected. The instrument is compara- 

 tively inexpensive, light, and easy to handle. Although it is de- 

 signed especially for measuring both dry- and wet-bulb temperatures, 

 it can, of course, be used to measure the air temperature alone, in 

 which case a reading of the wet-bulb thermometer is not necessary. 

 If the ship is equipped with a sling psychrometer, the observer will 

 always be able to measure the air temperature at the point which 

 happens to provide the best "free-air" exposure under the existing 

 conditions. The point of observation should always be on the wind- 

 ward side of the ship and preferably near the bow or the stern, 

 depending upon whether the apparent wind is forward of or abaft 

 the beam. It will usually be most convenient, however, to make the 

 observation on the wing of the bridge. The instrument should then 

 be held outside the rail and on the weather side. But if spray is 

 continually being blown aft from the bows, it will be necessary for 

 the observer to seek a more favorable location. 



It is very difficult to describe effectually the movements incident 

 tilated psychrometer as part of their regular meteorological equip- 

 ment. A comparatively rugged type (Bureau of Mines type) of 

 sling psychrometer, and one that is well fitted for use on shipboard, 

 is shown in figure 11. In this instrument the thermometer bulbs are 

 partially protected by a rectangular frame which encloses the other 

 parts of the apparatus. 



It is very difficult to describe effectually the movements incident 

 to skillful whirling and stopping of the sling psychrometer. The 

 arm is held with the forearm about horizontal, and the hand well 

 in front. A peculiar swing starts the thermometers whirling, and 

 afterward the motion is kept up by only a slight but very regular 

 action of the wrist, in harmony with the whirling thermometers. 

 The rate should be a natural one, so as to be easily and regularly 

 maintained. If too fast, or irregular, the thermometers may be 

 jerked about in a violent and dangerous manner. 



The stopping of the psychrometer, even at the very highest rates, 

 can be perfectly accomplished in a single revolution, when one has 

 learned the knack. This is only acquired by practice, and consists 

 of a quick swing of the forearm by which the hand also describes 

 a circular path, and, as it were, follows after the thermometers in 

 a peculiar manner that wholly overcomes their circular motion with- 

 out \h& slightest shock or jerk. The thermometers may, without very 

 great danger, be allowed simply to stop themselves ; the final motion 

 in such a case will generally be quite jerky, but, unless the instrmnent 

 is allowed to fall on the arm, or strikes some object, no injury should 

 result. 



5672°— 38 3 



