SPECIAL PROBLEMS 23 



readings about the critical hours, or, still better, with 

 the tracings of self-recording instruments. 



There are innumerable opportunities at sea for the 

 observation of other meteorological phenomena and 

 weather conditions for which no special instruments 

 are at present available, or, indeed, required. Such 

 phenomena, as being of only occasional occurrence, 

 shc'uld always be recorded, not because of their rarity 

 or their impressiveness, but because, as the times of 

 their appearing cannot be predicted, and few are found 

 ready when the phenomenon is suddenly recognized, 

 there are very few cases on record of exact measure- 

 ments. Among these are squalls, waterspouts, thunder- 

 storms, wind-borne dust, and optical and electrical 

 phenomena. 



Waterspouts should, if possible, be photographed, 

 the course of their formation and dispersion carefully 

 watched and described, and an endeavour made to 

 obtain angular measurements which will give the 

 distance and height of the columns and their rate of 

 movement. 



The optical phenomena include rainbows, rings round 

 the sun and moon (known as " coronse," " halos," 

 " paraselene," etc.), sun pillars, and the green ray. 

 These phenomena give the most striking effects at sea, 

 where they are seen much better than on land. The 

 sun shining on the drops of a passing shower produces 

 the rainbow by double internal reflection ; sometimes 

 a double or outer bow is. visible. Rainbows are often 

 to be seen at night, when the moon is particularly 

 bright. Coronae are due to rays of the sun or moon 

 shining through loose clouds of either the cumulus or 

 the stratus variety : they give the effect of faintly 

 outlined rings round the orb. Halos and the other 

 forms of ring are due to sunlight shining through 



