292 WELLS'S NATURAL PHILOSOPHY. 



be, a so-called equinoctial storm. The fact, however, should not be concealed, 

 that, taking the average for the five days embracing the equinox for the 

 period above stated, the amount of rain ia greater than for any other five 

 days, by three per cent., throughout the month. 



Observations recorded for a long period have proved that the phenomenon 

 of the aurora borealis, which is said to precede a storm, is as often followed 

 by fair, as by foul weather. 



Meteorologioal records, kept for eighty years at the observatory of Green- 

 wich, England, seem to show that groups of warm years alternate with coid 

 ones in such a way as to render it probable that the mean annual tempera- 

 tures rise and fall in a series of curves, corresponding to periods of about four- 

 teen years. 



There is little doubt that some animals and insects are able to foretell 

 changes in the weather, when man fails to perceive any indications of the 

 same. Thus some varieties of the land-snail only make their appearance be- 

 fore a rain. Some other varieties of land crustaceous animals change their 

 color and appearance twenty-four hours before a rain. 



For a light, short rain, some trees have been observed to incline their leaves, 

 BO as to retain water ; but for a long rain, they are so arranged as to conduct 

 the water away. 



The admonition given several thousand years ago, is equally sound in its 

 philosophy at the present day : " He that observeth the winds shall not sow ; 

 and he that regardeth the clouds shall not reap." — Eccles., xi. 4. 



CHAPTER XIV. 



LIGHT. 



whatisLightf ^^^- LiGHT is the physical agent which oc- 

 casions, by its action upon the eye, the sensa- 

 tion of vision. 



What is the 6'^^- Optics is the name given to that de- 

 sciencd of Op. pgrtment of physical science which treats of 

 vision, and of the laws and properties of light."* 

 Between the eye and any visible object a space of greater or less extent 

 intervenes. In some instances, as when we look at a star, the extent of the 

 space existing between the eye and the object seen is so great, that the mind 

 is unable to form any adequate conception of it. Yet we recognize the ex- 

 istence of objects at such distances, by the physical effect which they produce 

 on our organs of vision. 



• From the Greek word " Onrofiai" to see. 



