The answer is obvious, when we consider the various hydraulic motors at present in use.  Of course, motors worked by water pressure must here be excluded; and we are left with scarcely anything but the undershot wheel, the turbine, and the screw pump.  All these require expensive buildings and erections to set them to work, present but a very small fraction of their surface to the water at any one time, and must be very large and costly if they are to draw even a very moderate amount of power from such a source.  There is no possibility of adjusting them readily to suit variations in the speed of the current or in the quantity of water required, nor of moving them from place to place should this be convenient.

[Illustration:  PARACHUTE HYDRAULIC MOTOR.]

The motor of Mr. Jagn is on a totally different principle.  Its essential features consist, as shown, of an endless rope made of hemp or aloe fiber, which takes a turn or two round a pair of drums mounted on a barge or pontoon, and then passes down the channel to return over a pulley hung from a floating punt, at such a depth that the whole of the rope is immersed in the water.  Along this rope are suspended at equal intervals a number of parachutes made of sail cloth.  The rope passes through the center of each of these, and to it are attached a series of strings, the other ends of which are connected to the outside edge of the parachute.  Thus they act like the spokes of an umbrella to prevent the parachute from opening too far under the pressure of the current.  The parachutes must be placed so far apart that the current may act fairly on each, and the sum of the pressures forms the force which draws the rope through the water.  The moment, however, that any parachute has passed round the return pulley, the current acts upon it in the opposite direction.  It then shuts up like an umbrella, and assumes a volume so small that its resistance on the return journey is insignificant.  After passing round the drum at the upper end, it at once opens afresh of its own accord, and once more becomes part of the moving power of the whole system.  The parachutes are formed by first cutting out a complete circle of cloth, and then taking from this a sector equal to one-fifth or one-sixth of the total area.  Such parachutes are found to keep their form when stretched by the water better than a surface originally spherical, although the latter would be theoretically more correct.  The motion of the drum is transmitted by spur, gear, or otherwise as may be required, to give the requisite speed.

It will be seen that the advantages of the system are as follows:  First, the facility it offers for obtaining a large working area, which may be increased or diminished at will, according to the requirements of the moment, by lengthening or shortening the rope.  Secondly, the ease with which it is erected and set to work.  Thirdly, the small part of the river section which it occupies, so as to present no obstacle to navigation.  Fourthly, the ease with which it can be mounted on a barge of any kind, and carried wherever it may be needed.  Fifthly, it is not stopped, like all other hydraulic motors, by the appearance of ice—­it has, in fact, already been worked under ice in the Neva.  At the same time, winds and waves have no influence upon it.