PKEFACE. 



IN these days of limited leisure, and continually 

 increasing literary activity, when there is more and 

 more that every intelligent man would wish to read, 

 and apparently less and less time for reading, an apology 

 is required for the publication of a book, however 

 small, which does not profess to contain anything 

 absolutely new. At the same time, the very circum- 

 stances above referred to, render it desirable that 

 the observations of specialists should be condensed 

 and epitomized for the general reader, and hence 

 the continually increasing demand for- lectures ; which 

 need not, because they are popular, be the less truly 

 scientific. 



The fact that the present book does not contain any- 

 thing new to those who have specially studied the parts 

 of science with which it deals, precludes it from con- 

 stituting any tax on the time of those who have devoted 

 themselves to these subjects. 



