AMPLIFICATION OF THE LAW OF POI8EUILLE 51

V will not differ from — —;- — 2 by much over 0.2 per cent.

Zt

This means that working at atmospheric pressure, with a hydro-
static pressure of over 100 cm of water, one may take the volume

Vi + 72
of flow as - o — without any very appreciable error. It is

therefore extremely improbable that an appreciable error is
incurred through our lack of knowledge in regard to the exact
value of n in a given case. The effect of the temperature upon
the viscosity will be discussed later, page 246, as a temperature
correction.

The kinetic energy of the fluid increases as it passes along the
tube, but we are interested only in the total amount of thn
kinetic energy as the fluid leaves the tube. This is irp^R2!^.
The total energy supplied in producing the flow is 7rK2I2(Pi —Pz)g
and the difference between the two is the energy converted into
heat 7rR2h[(Pi — P%)g — qj[£\. The loss of head in dynes per
cm2 in imparting kinetic energy to the fluid is therefore

With this correction, but neglecting the slipping, we obtain

=
77

_
Z 2P2
Substituting V for F2 and remembering that pzVs is constant,
Eq. (19) becomes identical with the complete formula for the
viscosity as given in Eq. (17).
Although it is admitted that the flow of compressible fluids
is not quite linear, no correction for this has yet been attempted.
However it is certain that the correction is negligible if p is small
in comparison with P2. The correction for slipping in gases
plays an important part in the literature. The correction is the
same as for incompressible fluids.
Turbulent Flow in Gases. — The distinction between viscous
and turbulent flow in gases has been investigated by several
workers, among whom we may mention particularly Grindley
and Gibson (1908) and Ruckes (1908). Ruckes discovered that
the criterion for gases was greatly raised if the capillary was blown
out into a trumpet shape.