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.
Then there is the well-known spiral ventilator set in the windows of workshops. Perhaps its revolution may not assist the air-current, but it does, at all events, show how much exhausted air has to be expelled from the room, and consequently how much fresh air needs to be brought into it.
Perhaps the reader may be surprised to see that the Wings and Tail of a bird and a boy’s Kite are placed among the examples of the Spiral principle. Yet such is the fact. If the reader will move up and down the wings of any bird which will not bite him, he will find that there is in them a peculiar screwing motion, difficult of description, but very observable.
It is mostly for want of this movement that all our attempts at fitting wings to human beings have been such utter failures. We can make the wings work up and down well enough, but we cannot as yet impart to them the all-important spiral movement.
That very well-known toy, the Kite, is another example of the same principle which drives the screw steamer. Its “tail,” which need be nothing but a piece of string with a proportionate weight at the end, keeps the Kite in a slanting position, providing that the “belly-band” be properly arranged. The consequence is that the pressure of the wind acts on it as on a wedge, and so drives it upwards until the combined weight of itself and the string counterbalance the upward pressure.
Indeed, the only object of the string is to keep the Kite at a proper inclination; and, if that object could be attained by the force of gravity alone, the Kite would ascend to a height nearly double that to which it can at present attain.