XXIX
difference in level of different places is deter
mined (see Phy. Primer, page 25). It consists
simply of a metal or glass tube bent upward at
both ends. After having filled the tube with
water, the level at both ends will be the same,
and placing one's eye in a line with the two sur
faces, one is sure to look along a horizontal line
and is in a position to compare the heights of all
surrounding points with this level line. Still more
accurate is the Spirit-level. It is a glass tube,
which, with the exception of a bubble of air, is
filled with spirits. If in horizontal position, this
bubble will be between two points marked in the
tube. Both instruments are of the highest im
portance for the construction of canals, bridges,
railways, roads, etc.
§ 70. As to the Efflux of a liquid it cor
responds to the falling of a solid body and the
simple rule is this:
Through an orifice in the bottom or the side
(see § 71. a. Pascal's first law) of a vessel a liquid
flows out with a velocity equal to that attained by
a body during a free descent from the surface of
the liquid to the orifice of the vessel; c = 2 a. h
g (c = velocity; h = height; g = velocity at the end
of the first second). If the water flows out hori
zontally, it will describe part of a parabola. (Cf.
the horizontal projection § 51.)
Remark. The actual velocity of efflux will be somewhat less than
the theoretical one owing to a) adhesion at the orifice, b) the fact that there
is no absolute liquid (cf. page III, Remark), c) the falling water being
hindered by the particles of water aside.
