112 SCIENCE IN SHORT CHAPTERS. 



strikingly the yalue of the work of amateur astronomical 

 observers. 



By the collation and comparison of these, important in- 

 ductions are obtainable. Thus Professor A. S. Herschel 

 concludes that the earth passed through seven strata of 

 meteoric bodies, having each a thickness of about 50,000 

 miles in all about 350,000 miles. As the diameter of the 

 visible nebulosity of Biela's comet was but 40,000 miles 

 when nearest the earth in 1832, the great thickness of these 

 strata indicates something beyond the comet itself. 



Besides this, Mr. Hind's calculation for the return of 

 the primary comet shows that on November 27th it was 250 

 millions of miles from the earth. 



Those, however, who are determined to enjoy the sen- 

 sation of supposing that they really have been ' brushed by 

 the tail of a comet, still have the secondary comet to fall 

 back upon. This, as already described, was broken off 

 the original, from which it was seen gradually to diverge, 

 but was still linked to it by an arch of. nebulous matter. 



If this divergence has continued, it must now be far dis- 

 tant sufficiently far. to afford me an opportunity of safely 

 adding another to the numerous speculations, viz., that we 

 may, on November 27th, have plunged obliquely through 

 this connecting arm of nebulous matter, which was seen 

 stretching between the parent comet and its offshoot. The 

 actual position of the meteoric strata above referred to is 

 quite consistent with the hypothesis. 



THE "GKEAT ICE AGE" AND THE OEIGIN OF 

 THE "TILL." 



THE growth of science is becoming so overwhelming that 

 the old subdivisions of human knowledge are no longer 

 sufficient for the purpose of dividing the labor of experts. 

 It is scarcely possible now for any man to become a natu- 

 ralist, a chemist, or a physicist in the full sense of either 

 term; he must, if he aims at thoroughness, be satisfied 



