313 
SCW1CC ’ P * 50); and a S ain at P- 72 of the same work, he de- 
c aies that the question, Whence come we ? Whither go we ? dies without an 
‘ ' r ’ W1 .° UC GVen an echo ’ u P° n the infi nite shores of the Unknown in 
a work written four years later, he expresses his more mature thoughts in the 
o lowing candid way “In connexion with the charge of atheism I would 
make one remark. Christian men are proved by their writings to have their 
hours of strength and of conviction ; and men like myself share, in their own 
way, these variations of mood and tense. . . . But I have noticed durum 
years of self-observation that it is not in hours of clearness and vigour that 
this doctrine commends itself to my mind ; that in the presence of stronger 
and healthier thought it ever dissolves, as offering no solution of the mystery 
inwhich we dwell, and of which we form a part.” (Preface to the (itli 
edition of the Belfast Address, p. viii.) With regard to the letters from 
rotessors Birks and Challis, remarking on some portions of my paper ; 
entertaining, as I do, the highest opinion of those two distinguished pro- 
fessors of my own Alma Mater, I proceed to offer the following reply. 
Professor Birks objects to Mr. CrolPs theory, mentioned in° § 72, 
respecting the glacial period, and the excentricity of the earth’s orbit in 
bygone ages. Although I am quite ready to admit that it is only as yet 
an hypothesis, which must abide the test of time and investigation, yet I 
still think it the best mode of explaining the appearance of our coal- 
beds in high latitudes, where the flora of which they are composed could not 
exist with the present climate ; but I do not understand, as Professor Birks 
does, that Mr. CrolPs hypothesis respecting the glacial period being 
800,000 years ago, in any way affects the supposed antiquity of man. 
I understand Professor Birks’ objection to my assumption at § 73, 
to the supposed distance of the ££ fixed stars ” from our solar system, according 
to the theory of Herschel and Nichol, rests upon the disputed question, both 
m respect to the magnitude of the fixed stars, and also the full velocity of 
light, which depends upon the exact distance of the sun from the earth, whose 
mean distance is assumed to be 91,400,000 miles, but which may be here- 
after rectified by the calculations dependent upon the transit of Venus, which 
occurred in 1874, and will again take place in 1882. The Astronomer Eoyal 
of Scotland, however, speaks of this ££ merely as one step towards getting the 
sun-distance number perhaps a trifle better than before ” ; and he proceeds 
to call attention to the variations of science respecting the supposed distance 
of the sun in various ages of the world. Thus, of the learned Greeks, Hero- 
dotus supposed the sun to have been a mere satellite of the earth, acted upon 
by the same forces which are sensible to us (lib. ii. § 24), and consequently 
could only have been distant about ten miles. Anaxagoras computed it at 
about 14,000 miles. Aristarchus increased it to over 5,000,000 miles. Two 
thousand years later, Kepler enlarged it to over 26,000,000. Delambre, in 
the eighteenth century, advanced it to 90,100,000 miles. Since that time, 
the distance in mileage has been gradually receding, until Henderson, in 
1832, reduced it to 89,586,000 miles. Since then, — “ the real sun-distance by 
VOL. X. z 
