THE "GREAT ICE AGE.'* 157 



A multitude of letters were read at the last and previous 

 meeting of the Astronomical Society, in which the writers 

 described the details of their own observations. As these 

 letters came from nearly all parts of the world, the data have 

 an unusual degree of completeness, and show very strikingly 

 the value of the work of amateur astronomical observers. 



By the collation and comparison of these, important induc- 

 tions are obtainable. Thus, Professor A. S. Ilerschel 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 2*7th it was 250 

 millions of miles from the earth. 



Those, however, who are determined to enjoy the sensation 

 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 distant 

 sufficiently far to afford me an opportunity of safely adding 

 another to the numerous speculations viz. that we may, on 

 November 2Yth, have plunged obliquely through this connect- 

 ing arm of nebulous matter, which was seen stretching 

 between the parent cornet and its offshoot. The actual posi- 

 tion of the meteoric strata above referred to is quite consistent 

 with this hypothesis. 



CHAPTER XXIII. 



THE " GREAT ICE AGE" AND THE ORIGIN 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 naturalist, a chemist, or 

 a physicist in the full sense of either term ; he must, if he 



