an hour later both ends rose from
their foundations. When the tide had risen 4 ft. the stage and bridge
were floated to the new position, when at 8.30 the girders dropped on
to their beds.

* * * * *

THE GENERATION OF STEAM, AND THE THERMODYNAMIC PROBLEMS
INVOLVED.[1]

[Footnote 1: Lecture delivered at the Institution of Civil
Engineers, session 1883-84. For the illustrations we are indebted
to the courtesy of Mr. J. Forrest, the secretary.]

By Mr. WILLIAM ANDERSON, M.I.C.E.

It will not be necessary to commence this lecture by explaining the
origin of fuel; it will be sufficient if I remind you that it is to
the action of the complex rays of the sun upon the foliage of plants
that we mainly owe our supply of combustibles. The tree trunks and
branches of our forests, as well as the subterranean deposits of coal
and naphtha, at one time formed portions of the atmosphere in the form
of carbonic acid gas; that gas was decomposed by the energy of the
solar rays, the carbon and the oxygen were placed in positions of
advantage with respect to each other--endowed with potential energy;
and it is my duty this evening to show how we can best make use of
these relations, and by once more combining the constituents of fuel
with the oxygen of the air, reverse the action which caused the growth
of the plants, that is to say, by destroying the plant reproduce the
heat and light which fostered it. The energy which can be set free by
this process cannot be greater than that derived originally from the
sun, and which, acting through the frail mechanism of green leaves,
tore asunder the strong bonds of chemical affinity wherein the carbon
and oxygen were hound, converting the former into the ligneous
portions of the plants and setting the latter free for other uses. The
power thus silently exerted is enormous; for every ton of carbon
separated in twelve hours necessitates an expenditure of energy
represented by at least 1,058 horse power, but the action is spread
over an enormous area of leaf surface, rendered necessary by the small
proportion of carbonic acid contained in the air, by measure only
1/2000 part, and hence the action is silent and imperceptible. It is
now conceded on all hands that what is termed heat is the energy of
molecular motion, and that this motion is convertible into various
kinds and obeys the general laws relating to motion. Two substances
brought within t