WHAT IS KNOWN OF THE EARTH. 8ii 



and others extending, with no apparent tendency to periodicity, 

 over considerable lengths of time. The manner in which these 

 variations occur is still a matter of investigation, and their causes 

 are doubtful, but the diurnal and annual changes are probably 

 connected with changes of the temperature of the earth or its 

 atmosphere, and may be influenced by geographical conditions. 

 The non-periodical changes that have been recorded are very 

 large. These variations have been attributed by some to changes 

 going on in the condition of the interior of the earth, and by others 

 to external influences ; but they continue to be among the most 

 obscure of physical phenomena. Besides the variations above 

 mentioned, there also arise other irregular disturbances of the 

 indications of the magnetic needle, of short duration, which are 

 sometimes spoken of as magnetic storms. They occur with a fre- 

 quency which shows a tendency to periodicity, diurnal or annual, 

 and often almost simultaneously at distant parts of the earth, with 

 nearly identical effects, and with a marked increase in intensity 

 with increase of latitude. They likewise exhibit a period of in- 

 crease and decrease coinciding with that observed in the sun-spot 

 area, thus giving additional reason to connect them with modi- 

 fications of the magnetic or electric condition of the earth or 

 atmosphere arising in some manner from the action of the sun. 

 The probable connection of these disturbances with the electrical 

 condition of the atmosphere is indicated by their frequent occur- 

 rence simultaneously with appearances of the aurora, and with 

 electrical earth-currents. The frequent, if not continuous, display 

 of the aurora in the vicinity of the magnetic poles, further sug- 

 gests a relation between the electrical and magnetic conditions of 

 the earth. The true nature of all these phenomena is, however, 

 still very imperfectly ascertained. 



A very little olieervation and thought threw discredit on the 

 ancient cosmogonies, and showed that they failed to give any sat- 

 isfactory solution of the problems submitted by the advance of 

 knowledge. If the extravagant myths of Asiatic origin, which 

 peopled the earth millions of years ago with races of anthropo- 

 morphic demigods, and heroes descended from the sun and moon, 

 could not bear the test of facts, neither have those traditions 

 fared better which unveil the earth fully equipped with all the 

 present forms of life and specially prepared to be the dwelling- 

 place of man, some few thousand years ago. Precise observation 

 has now supplied satisfactory proof that the earth's surface, with 

 all that is on it, has been evolved through countless ages, by a 

 process of constant change. Those features that at first sight ap- 

 pear most permanent, yet in detail undergo perpetual modifica- 

 tion, under the operation of forces which are inherent in the ma- 

 terials of which the earth is made up, or are developed by its 



