Guesses at the Vast 19 



a planet, but the conclusion is always in the nature of an 

 inference more or less probable; or if you like, it must always 

 be a " guess." The available facts are those that the astrono- 

 mer can find in the sky — the behavior of suns, that is, 

 stars, and nebulas. There are also facts to be observed upon 

 the earth and within the earth. From these facts various 

 theories have been developed as to the probable course of 

 events that led to the present constitution of the earth. It 

 may be said at once that no theory is altogether satisfactory 

 since none takes account of all the known facts; and perhaps 

 each theory is at some point inconsistent with known facts. 

 It should be added, however, that this in no way discredits 

 these theories since they have served a very useful purpose 

 in guiding scientists in the collection of facts, in making 

 observations, and in checking or criticizing one another's 

 observations. 



Guesses at the Past 



In trying to understand what happened during the early 

 history of the earth, we have assumed that the happenings 

 in any case were the same as those which occur today tinder 

 similar eirctcmstances. For example, What does water do 

 to different kinds of materials? How rapidly does water 

 carry sand away? How rapidly does sand settle at the mouth 

 of a river or in a lake? How rapidly can rivers carry away 

 dissolved materials from the soil? Rain falling upon bare 

 soil or sand washes the particles down and carries them along 

 in its course (Fig. i ) . Water soaking into the soil dissolves 

 certain substances, leaving others undissolved. The water 

 gathering into brooks and rivers finds its way at last into 

 the ocean. Here the heat of the sun brings about evapora- 

 tion of the water but leaves behind the dissolved solids, 

 whereas the suspended or floating particles settle to the bot- 

 tom. The scientist assumes that water has always behaved 

 in these ways with relation to sand or soil, with relation to 

 soluble substances, with relation to heat. 



