26 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



thermometers inside it, unless they are cleaned from time to time, 

 will become coated with a thin deposit of salt. Thus the wet bulb, 

 though pure water be applied to it, will in reality be moistened by 

 a weak salt solution. Since dissolved salt has the effect of decreasing 

 the rate of evaporation, the temperature indicated by the wet-bulb 

 thermometer wdll be somewhat higher than the true wet-bulb tem- 

 perature. Furthermore, the problem of subjecting thermometers that 

 are housed in a shelter (portable or otherwise) to the proper ventila- 

 tion necessary for accurate humidity measurements presents obvious 

 difficulties. Finally, there is always the clanger that the shelter it- 

 self may be smashed or carried away in heavy weather, especially if 

 it is mounted (as it ought to be, in order to secure true air tempera- 

 tures) in an exposed position. 



The conclusion is that a thermometer shelter, though it serves its 

 purpose satisfactorily on land, is not well suited for use on ship- 

 board. Therefore, it is desirable, whenever possible, to employ some 

 special type of instrument wdiose readings of air temperature will 

 not be affected by direct exposure of the thermometers to sunshine 

 and rain. 



The aspiration thermometer. — The instrument best adapted to the 

 accurate measurement of air temperatures at sea is the portable 

 aspiration thermometer. Its essential features are (1) protection of 

 the thermometer bulb from radiation (direct sunshine or deck radi- 

 ation) by means of a tubular shield, made of a good reflecting and 

 insulating substance; (2) artificial ventilation of the thermometer 

 bulb, which is produced by a mechanicallj' driven fan attached to 

 the top of the instrument. The chief advantages of the aspiration 

 thermometer are three-fold. Exposure to direct sunshine will not 

 affect its readings. Consequently there is no necessity for placing 

 it in the shade at the time an observation is made. When not in 

 use it may be hung in the chart room or other convenient place 

 where it will be protected from the effects of fog, rain, and spray. 

 Finally, when an observation is made, the observer can readily select 

 the most suitable exposure under the conditions prevailing at any 

 given hour of observation and make the reading directly at that 

 point. 



For the measurement of temperature alone there is available a 

 type of aspiration thermometer (dry bulb only) that is rugged and 

 especially constructed for use at sea. This instrument is compara- 

 tively inexpensive. 



PSYCHROMETERS 



From the foregoing considerations it follows that, purely from the 

 standpoint of accuracy, the ideal method of obtaining simultaneously 

 the true air temperature and humidity on shipboard is the use of an 

 aspirating instrument that is equipped with two thermometers — one 

 a dry bulb, the other a wet bulb. Such an instrument is known as 

 an aspiration psychrometer. HoAvever, the type now available is 

 both expensive and delicate. It is also rather heavy and difficult to 

 handle, especially on a ship that is rolling or pitching. Further- 

 more, to keep it in proper condition requires a great deal of care and 

 attention. From a practical standpoint, therefore, it is not really 

 suitable for makinc: routine observations at sea. 



