500 



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 conse- 

 quently 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 propor- 

 tionate weight at the end, keeps the Kite in a slanting posi- 

 tion, 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. 



CENTRIFUGAL FORCE. 



CLOSELY connected with the spiral principle is Centrifugal 

 Force, that marvellous power which gives to our whole solar 

 system its ceaseless movements, and may extend, as far as we 

 know, to other and vaster systems yet unknown. 



Tie a ball to a string, and swing it round, and it will be an 

 exact, though rough, representation of the double power by 

 which the movements of the heavenly bodies are governed, our 

 earth being included among them. 



