As a rule, it keeps up a rotation of tentacles, such as is shown in the illustration, acting exactly on the principle of the Turbine Pump, and drawing in and discharging water with a power that is perfectly astonishing in so small a being. Beside this, it has the power of flinging out at once the whole of its watery contents, and any one who has incautiously handled a mass of Ascidians, and been drenched by them, can answer with more truth than satisfaction as to the water-absorbing power of the Turbine.
Then the Ascidian can do what the Turbine cannot do. In the Turbine the water which is taken in must necessarily be ejected in equal proportions. With the Ascidian the same thing takes place, but with the additional power of ejecting all the contained water, and then beginning afresh.
There is now no doubt that the Circular or the Turbine Pump is the most powerful in such cases as emptying mines of the water which, in spite of all precautions, will make its way in, and destroy the labours of the miners. But I merely wish to carry out the object of this work by remarking that the invaluable Turbine Pump is only a very inferior copy of a natural pump, which existed, as far as we know, centuries before Man could find his place upon this earth.
The Spiral.
In an early portion of this work the Spiral or Screw was touched upon, mostly in connection with the propulsion of vessels. We will now extend it a little further, and see how it is modified so as to perform other offices than those which have been described.
Allusion has already been made to the Spiral or Wedge principle, but some of the illustrations were accidentally omitted. I therefore introduce them here, this being a chapter of miscellanea.
The Windmill has previously been described, as has also the ship’s Screw, another form of which is here given.
In the centre is shown the mechanism popularly known as the Smoke-jack, though it really works by means of hot air, and only becomes gradually choked by the soot which the smoke by degrees deposits upon it. It is, in fact, nothing but a windmill working horizontally instead of vertically, the vanes being moved by the rapidly ascending heated air. So powerful is the spiral pressure of this air, that in my old college days at least a dozen rows of heavily laden spits were perpetually turned by a single Smoke-jack. It is many years since I visited my old college, and I cannot say whether the Smoke-jack still exists, but, as it did its work well so long ago, I presume that it does so now.