22 THE AIR 



involving direction the utmost care must be taken to 

 state clearly whether each reading is magnetic or true, 

 and, if magnetic, the true bearing should be calculated 

 and entered as soon as the variation is known. Much 

 trouble has arisen through the neglect of this very- 

 obvious precaution. 



Special Problems in Meteorology. 



Meteorological observations at sea on a scientific 

 expedition are of great importance with reference to 

 oceanographical observations, such as the temperature 

 of the surface water and of the deeper layers, the 

 surface currents, the occurrence of plankton, and other 

 matters. But it is also important to consider meteor- 

 ological observations from the point of view of our 

 knowledge of the science of meteorology itself, and 

 there are some problems which are more likely to be 

 elucidated by observations at sea than by those on 

 land. One of these is the angle with the horizontal 

 made by the wind when it strikes the sea and the effect 

 of winds of different degrees of obliquity. So far as 

 we know, nothing has been done in this direction ; but 

 it is obviously of importance, with regard both to 

 wind-raised waves and wind-driven currents. How 

 such observations can be made we must leave to the 

 ingenuity of the investigator, whose attention has been 

 called to the subject. 



Another matter, concerning which more information 

 is required, is the diurnal variation of phenomena. The 

 hours at which the two maxima and minima of pressure 

 and the maximum and minimum temperature occur in 

 each diurnal period can be determined only roughly 

 from routine observations, and it is worth while to 

 supplement the regular readings with more frequent 



