377 
at our nearest to the sun in the earth's eccentric orbit. He 
argues only for a “ perpetual spring." His mean temperature, 
calculated for Great Britain, is only 60° F. This, he argues, 
must have been the summer and winter heat, with scarcely any 
variation, in the Carboniferous period. But, as we have seen, 
geology calls for the climate of the hottest parts of India, an 
equatorial climate whose mean heat is 81°. What we want is, 
at least, a tropical climate in the latitude of London — a climate 
very different indeed from that which, even according to re- 
vised ideas, could suit the vegetation of the Coal period. In 
thinking of the possibilities of such a climate in the North, it 
is necessary to keep in mind the truth to which we have 
already referred, that the length of the polar summer's day, 
though giving great advantage in the reception of heat by 
the constantly enlightened parts, presents only a slanting face 
to the sun, and so can never account for the heat and other 
effects which flow from the vertical radiance of Bengal. Sir 
Charles Lyell, in criticising Mr. Cr oil's theory, quotes from 
the Encyclojocedia Britannica , the results of the reasoning 
there given in the article on climate. It is to the effect that 
the sun's rays passing through the atmosphere, so as to fall on 
the earth's surface at the equator, give 115° of heat, for 51° 
given in latitude 45° south or north, and for 14° given at 
either pole.* The latitude of the London clay is 51° 30' N. 
The radiance of the sun, which gives 115° F. at the equator, 
and gives only 51° as far as 45° north latitude, is re- 
quired to give an equatorial heat more than six degrees 
further north than where it can give only 51°. How will 
Mr. Croll, or any one else, make this out, and so explain on 
this theory the tropical remains in the isle of Sheppey ? Yet 
this is that for which an account is required as the facts of 
geology stand. 
The remains which, as we have seen, are imbedded in the 
London clay and kindred formations, are such that nothing 
short of the sun's vertical radiance will account for them. 
Hr. Hook saw this as early as 1688, and although his 
idea has been scouted, it is not on that account the less 
true. But, in addition to all this, any one who has had to do 
with the growth of palms and other tropical plants in this 
country, knows that it is not so much want of heat which ren- 
ders it impossible to grow them satisfactorily, nor is it the 
want of moisture. These can be supplied ; but what we lack 
is the sun's tropical radiance. Sunshine means much more 
than mere heat. How to show that this ever fell on the 
* Lyell’s Principles of Geology, vol. i., edition 1867, p. 284. 
2 F 
