79 
balancing the force by the resistance at the foot of the falls 
is just as open to view as the falling force itself ; so that the 
motions, too small to be perceived, are supererogatory inven- 
tions. The dynamical origin of heat is also set forth by 
Professor Tyndall, in a paper “ On the Forms and Action of 
Water.” Examples are therein adduced of vast mechanical 
forces exerted by the formation of water, and by its changes 
from the states of a solid, liquid, and vapour. He says that 
“ on the combination of oxygen and hydrogen to form water 
weighing 101b. an energy was expended, the atoms clashing 
together with a force equal to that of a ton weight let fall 
from a height of 2-3,757 feet. In falling from the state of 
vapour to that of water an energy was expended equal to that 
of a ton falling from a height of 3,700 feet, the moving force 
of the stone avalanches of the Alps was but as that of snow- 
flakes, compared with the energy involved in the formation 
of a cloud. In passing finally from the liquid to the solid 
state, the atoms of 101b. of water exercised an energy equal to 
that of a ton weight falling down a precipice 550 feet high.” 
Now, the union of the gases to form water, the evapora- 
tion of it to form steam, and freezing it into ice, are chemical 
changes, and the forces called into action by them are chemi- 
cal forces, which cannot be measured by that of gravity in 
falling tons or even ounces ; but the mutations of heat 
attending those changes are measured by degrees, when 
passing from the latent to the sensible state, and vice versa. 
Of vapour in the air, the Professor says, that “ though 
forming only about 0'5 per cent of the atmosphere, for every 
ray of terrestrial heat struck down by the air, fifty, sixty, or 
seventy rays were destroyed by the aqueous vapour. This 
vapour permitted the solar rays to reach the earth, but inter- 
cepted the terrestrial rays in their escape towards space,” and 
he adds that “ in the presence of such experiments, it was easy 
to see how the snow of the Alpine mountains, and how the ice, 
should be squeezed through the moulds formed by the valleys, 
