196 THE POPULAR SCIENCE MONTHLY. 



ever, been comparatively recent,. and is in a great measure due to the 

 teachings of that eminent geologist, the late Sir Charles Lyell, whom 

 we have lost during the past year. 



When we look back by the help of geological science to the more 

 remote past, through the epochs immediately preceding our own, we 

 find evidence of marine animals which lived, were reproduced, and 

 died possessed of organs proving that they were under the influence 

 of the heat and light of the sun ; of seas whose waves rose before the 

 winds, breaking down clifis, and forming beaches of bowlders and 

 pebbles ; of tides and currents spreading out banks of sand and mud, 

 on which are left the impress of the ripple of the water, of drops of 

 rain, and of the tracks of animals ; and all these appearances are pre- 

 cisely similar to those we observe at the present day as the result of 

 forces which we see actually in operation. Every successive stage, 

 as we recede in the past history of the earth, teaches the same lesson. 

 The forces which are now at work, whether in degrading the surface 

 by the action of seas, rivers, or frosts, and in transporting its frag- 

 ments into the sea, or in reconstituting the land by raising beds laid 

 out in the depth of the ocean, are traced by similar effects as having 

 continued in action from the earliest times. 



Thus pushing back our inquiries we at last reach the point where 

 the apparent cessation of terrestrial conditions such as now exist re- 

 quires us to consider the relation in which our planet stands to other 

 bodies in celestial space ; and, vast though the gulf be that separates 

 us from these, science has been able to bridge it. By means of spec- 

 troscopic analysis it has been established that the constituent elements 

 of the sun and other heavenly bodies are substantially the same as 

 those of the earth. The examination of the meteorites which have 

 fallen on the earth from the interplanetary spaces shows that they 

 also contain nothing foreign to the constituents of the earth. The 

 inference seems legitimate, corroborated as it is by the manifest phys- 

 ical connection between the sun and the planetary bodies circulating 

 around it, that the whole solar system is formed of the same desci'ip- 

 tions of matter, and subject to the same general physical laws. These 

 conclusions further support the supposition that the earth and other 

 planets have been formed by the aggregation of matter once diffused 

 in space around the sun ; that the first consequence of this aggrega- 

 tion was to develop intense heat in the consolidating masses ; that 

 the heat thus generated in the terrestrial sphere was subsequently 

 lost by radiation; and that the surface cooled and became a solid 

 crust, leaving a central nucleus of much higher temperature within. 

 The eartli's surface appears now to have reached a temperature which 

 is virtually fixed, and on which the gain of heat from the sun is, on 

 the whole, just compensated by the loss by radiation into surround- 

 ing space. 



Such a conception of the earliest stage of the earth's existence is 



