Cuar. IV., § 6.) MECHANICS (OF FLUIDS).—MR RUSSELL—M. FOURNEYRON. 
pear whether the disturbed or undisturbed depth is 
Compared to betaken. The analysis of Mr Airy seems to show 
with theory that a depth somewhat greater than that due to the 
utmost effect of disturbance is to be preferred. Mr 
Mr Airy. Russell also made experiments on the propagation of 
waves in channels of different forms of section. Pro- 
fessor Kelland has given a very simple expression 
for the velocity of the wave in this case,' which on 
the whole agrees with experiment. 
(422.) Sometime about the year 1830, attention was 
phe drawn. to a singular fact connected with the resist- 
wavetrans- ance of water in the case of canal navigation. It 
mission on was first noticed I believe in Scotland, probably on 
canal navi-the Forth and Clyde Canal. It amounted to this, 
gation. that whereas at moderate or rather slow velocities, 
the resistance to a boat increases with the square 
of the velocity,—after a certain point, not differing 
very much from 7 miles an hour, the resistances not 
only cease to increase according to the same rapid 
law, but actually diminish to some extent when the 
speed is greater. Different experiments were made 
by the canal proprietors with a view to meet in some 
degree the active competition by railways, then com- 
mencing. Mr Russell was employed by the directors 
of the Forth and Clyde Canal, and to his experiments 
we now refer. It appears from his tables that the 
resistances increased on the whole faster than the 
squares of the velocities up to 7} miles an hour, 
when they suddenly diminished between 7} and 8} 
miles an hour by one-jifth part in one experiment, 
and by no less than one-third in another.? It was 
not until about 12 miles an hour, that the resistance 
reached the same amount as at 74 miles. 
(423.) The occurrence of this singular transition was at- 
gy tended with a phenomenon easily noticeable. In 
every ordinary case a boat in a canal drives a wave 
before it, which is in fact a heap of water resisting 
the boat by increasing the pressure against its bows, 
which wave may be called a forced wave, having this 
peculiarity that it travels with the speed of the boat 
and never quits it; whilst a free wave, by whatever 
cause excited, is propagated at arate depending only 
on the dimensions of the canal, particularly its depth. 
Now the diminished resistance takes place when the 
boat is by the force of traction partly drawn out of the 
water, and lifted up upon the wave to which its own 
motion gives rise. It is said to ride upon the wave, 
and the head of water pressing against its bows is 
visibly diminished. The most advantageous rate of 
transport was found to be about one-third greater than 
that required merely to mount the wave, which last 
depends principally on the depth of the canal. Thus 
on three different canals, 3}, 5}, and 9 feet deep, 
the most advantageous velocities were 8, 11, and 
15} miles an hour. The actual velocities of the 
free wave were ascertained by Mr Russell in an in- 
889 
genious and satisfactory manner. When the boat is 
dragged to most advantage, the draught of water is 
less at the stem and stern than in the centre. All 
these circumstances have been very ingeniously and 
satisfactorily explained by Mr Airy in his paper on 
Tides and Waves, articles (404-411). 
Many persons (amongst whom are Colonel Henry 424.) 
Beaufoy, Mr Scott Russell, and the American ship- Forms of 
builders) have bestowed much attention on the forms *P'P* 
of vessels for ensuring speed, especially by the avoid- 
ance of waves of various kinds generated by steam- 
vessels in motion. Every one who can compare the 
performance of such vessels during the last twenty 
years, and the still surface which waters navigated 
by steam vessels now present, as if they were merely 
cut open and closed again before and after the passage 
of the ship, instead of being tossed into dangerous bil- 
lows consuming uselessly the propelling force, will 
readily admit that, however imperfect the theory, prac- 
tical art has made real progress in this direction. 
ILI. Improved Hydraulic Machines—Turbine.—  (495,) 
Before concluding this section, I will refer to the Improved 
most considerable improvement made of late years bydvsulic 
in the application of hydraulic pressure to motive jj. Tus. 
purposes, and I shall couple it with the names bine. 
of two French engineers, MM, Fourneyron and 
Poncetet, the former the inventor of the Turbine 
(the machine referred to), at least in its improved 
practical form; the latter an important contributor 
to the useful application of hydraulics, an accom- 
plished mathematician, and the author of several 
standard works connected with industrial mechanics. 
The defects of common vertical water-wheels, (426,) 
whether overshot or undershot, are so great and Defects of 
so notorious, that only their simplicity, and the Son 
fact that in very many cases water-power costs whoels— 
next to nothing, and may be squandered with im- Barker’s 
punity, could justify their use, The advantage of ™ill- 
using the simple pressure of a fluid as a moving 
power had been foreseen in that application of re-ac- 
tion called Barker’s Mill; which, though well known 
in models, was seldom if ever applied in practice. 
Mathematicians were, however, aware that it offered 
important advantages. Of late years a patent has been 
taken out in Scotland for a modification of it, which 
is found, I believe, to work well. But the Turbine 
or horizontal water-wheel imagined by Burdin and 
Fourneyron, and brought to a high state of perfection 
by the latter about the year 1833, appears to exhaust 
all that is valuable in this mode of applying water. 
Referring to other parts of the Encyclopedia 497.) 
for the details, I may here explain generally that Fourney- 
the Turbine consists of two parts, one a fixed cy- [ens tue 
linder or drum of small height compared to its  ~ 
diameter; the other a portion of a cylinder ex- 
terior to the former, and moveable round it, so that 
1 Velocity = V ? “ 3 where.A is the area of section of the canal, } the breadth of the water at the surface, and g the accele- 
rating effect of gravity. Edinburgh Trans. vol. xiv. 
® Edin. Trans., vol. xiv., p. 48; 
VOL. I. 
5u 
