THERMOMETERS 15 



checked by periodical readings of the mercurial 

 barometer . Specially sensitive patterns, provided with 

 a drum revolving more rapidly than in the ordinary 

 form, may be used to keep a record of the height of the 

 waves encountered. 



3. Temperature is measured by thermometers, which 

 should be of standard type, tested at Kew or at some 

 other national testing establishment, and supplied with 

 a certificate of accuracy. This condition is insisted 

 on for all the instruments supplied by the Meteoro- 

 logical Office for use at sea. The wet and dry bulb 

 thermometers must be exposed in a louvred screen, 

 quite in the open, free from all artificial heat, and pro- 

 tected from sea-spray and from all risk of injury. The 

 bulb of the dry thermometer must be kept as free from 

 moisture as possible, and the bulb of the wet ther- 

 mometer free from salt. The object of the wet bulb 

 thermometer in conjunction with the dry bulb is to 

 supply data for calculating the humidity of the air, for 

 which purpose tables are printed in all books of 

 instructions for the use of meteorological instruments. 



The wet and dry bulb thermometers, used for 

 measuring the temperature and humidity of the air, 

 act as well at sea as on shore ; but the water supplied 

 to the wet bulb and the muslin covering the bulb must 

 be more frequently changed, to prevent incrustation 

 with salt. A weekly change of muslin and the use of 

 distilled water would probably be found necessary, in 

 order to obtain accurate results. A more serious 

 difficulty which arises is the totally inadequate char- 

 acter of the " Board of Trade screen " usually supplied 

 for housing the thermometers. This is too small, and 

 its place should be taken by an ordinary Stevenson 

 screen, as employed on shore. The great size of modern 

 steamers gives ample room for setting up such a screen, 



