48 
COURSE OF THE SAP. 
[Part II. 
cause, and laws, and mechanism by which the 
sap is forced up. But there I should be wrong. 
Every one has his own idea, and every one’s idea 
differs from that of his neighbour. To show on 
what extraordinarily loose ground these ideas 
stand, I will quote two countenanced by per¬ 
haps the keenest intellect that ever wrote on 
the subject. Sir Humphrey Davy thought that 
one cause of the ascent of the sap was the mo¬ 
tion caused in trees by wind; that another 
cause was the contraction and expansion of the 
wood from alternation of heat and cold. Look 
into the hot-house and the hot-bed. In these 
neither of these causes exists. Not a breath of 
wind enters; nor is any alternation of heat and 
cold allowed. Yet in these the ascent of the sap 
is freest. And if we look out of doors, I should 
say that the sap would be a slow traveller, if its 
ascent depended on wind and cold. Here, then, 
I cannot back the favourite, and have a sort of 
blind leaning for Turgescence , or Swelling. A 
dark horse, certainly! and I am all in the dark 
about him myself. 
Perhaps the largest blocks of stone ever 
quarried by man (I do not except Pompey’s 
Pillar for length) are from the granite quarries 
in Finland. One mode of rending these from 
their beds was to drill holes along their sides, 
