322 PHYSICAL SCIENCE 



the star-like balls of light, which in stormy weather are sometimes 

 seen on the masts of vessels at sea, and which before his time 

 had been observed on the points of the spears of an army in the 

 field. This luminous appearance, regarded by the Romans as a 

 sign of the friendly presence of Castor and Pollux, is entirely 

 atmospheric, and has no connection with shooting stars. It is 

 now known as St. Elmo's Fire, and has been shown to be a gentle 

 continuous electric discharge from the earth towards a cloud. 



Seneca next describes in some detail a series of optical appear- 

 ances connected with the sun and moon. Until the laws of the 

 reflection and refraction of light had been discovered, it was 

 obviously impossible to account for these phenomena. There is, 

 therefore, much interest in following the lines of thought by which 

 the old philosophers attempted to explain them. Seneca clearly 

 perceived that the halos and coronae seen round the sun and 

 moon in certain states of the atmosphere do not belong to these 

 luminaries, but to our own air, and may furnish indications of 

 coming weather. He remarks shrewdly enough that appearances 

 akin to those seen in the sky may sometimes be observed in the 

 thick moist air of a bathroom. But when he confidently proceeds 

 to explain the meteorological phenomena he betakes himself to 

 analogy, as he is so fond of doing. He remarks that when a 

 stone is thrown into a pond a succession of circles is produced 

 on the surface of the water, which continually widen from the 

 point of impact until they lessen and disappear. In like manner 

 he believes that when the light of the sun or moon strikes the 

 cloudy air it produces a similar effect, for as every kind of light 

 is round in shape, the air is thus driven into a circular form. 

 His love of analogy generally, as in this instance, leads him far 

 away from the truth, and prevents him from seeing the palpable 

 flaws in his reasoning. But the apparent similarity of appearances, 

 which are in reality entirely dissimilar, contents him with his 

 explanations. 



His discussion of the rainbow (16-33) ^ s one f ^ ne m st 

 detailed and vivacious in the whole volume. It takes the form 

 of a sustained argument, in which the author cites various authori- 

 ties, and replies to objections brought by a supposed opponent 

 to his thesis, which is that the rainbow is unquestionably an image 

 of the sun received in a very moist cloud which has the shape 

 of a round concave mirror (20, 27). He quotes with apparent 

 approbation the opinion that in a shower of rain each falling drop 

 is a mirror reflecting an image of the sun, and that when an 

 observer stands directly between the sun and the shower he sees 

 the reflections of the countless drops blended into one continuous 

 semicircle. But as the discussion proceeds the writer denies that the 



