er, tending to produce a vacuum there, this
water is pushed up by the pressure of the air upon the water outside the
cylinder, and follows the rising piston, until the column of water
inside the cylinder exerts a pressure equal to that of the atmosphere
upon an equal area. So much for the computation; does it correspond with
the fact? It is found that at the sea level water can be pumped to the
height of 33 ft; and that such a column of water has a pressure of 15
lb. to the square inch. We may show further that, at the sea level,
spirits of wine may be pumped higher according to its less specific
gravity; and that if we attempt to pump water at successive altitudes
above the sea level, we can only raise it to less and less heights,
corresponding with the lessened atmospheric pressure at those altitudes,
where the column of air producing the pressure is shorter. Finally, if
we try to work a pump, having first produced a vacuum over the water
outside the cylinder, we shall find that the water inside will not rise
at all; the piston can be raised, but the water does not follow it. The
verification thus shows that the computed effect corresponds with the
phenomenon to be explained; that the result does not depend upon the
nature of water only, but is true (allowing for differences of specific
gravity) of other liquids; that if the pressure of the outside air is
diminished, the height of pumping is so too (canon of Variations); and
that if that pressure is entirely removed, pumping becomes impossible
(canon of Difference).

Any text-book of Astronomy or Physics furnishes numerous illustrations
of the deductive method. Take, for example, the first chapter of
Deschanel's _Optics_, where are given three methods of determining the
velocity of Light. This was first deduced from observation of Jupiter's
satellites. The one nearest the planet passes behind it, or into its
shadow, and is eclipsed, at intervals of about 42-1/2 hours. But it can
be shown that, when Jupiter and the Earth are nearest together on the
same side of the Sun, an eclipse of this satellite is visible from the
earth 16 min. 26.6 sec. earlier than when Jupiter and the earth are
furthest apart on opposite sides of the Sun: 16 min. 26.6 sec, then, is
the time in which light traverses the diameter of the Earth's orbit.
Therefore, supposing the Earth's distance from the Sun to be 92 millions
of miles, light travels about 186,000 miles a second. Another deduction,
ag