ON AERIAL LOCOMOTION. 300 



it will be found that, for tlie purpose of effecting the niaxiniiun amount 

 of resistance, the more rapidly the spindle revolves the narrower may 

 be the blade. There is a specific ratio between the width of the blade 

 and its velocity. It is of some importance that this should be precisely 

 defined, not only for its practical utility in determining^ the best pro- 

 portion of width to speed in the blades of screw-propellers, but also for 

 a correct demonstration of the principles involved in the subject now- 

 under consideration; for it maybe remarked that the swiftest-flying- 

 birds possess extremely long and narrow wings, and the slow, heavy 

 flyers short and wide ones. 



In the early days of the screw-propeller it was thought requisite, in 

 order to obtain the advantage of the utmost extent of surface, that the 

 end view of tlie screw should present no opening, but appear as a com- 

 plete disk. Accordingly, some were constructed with one or two 

 threads, making an entire or two half-revolutions ; but this was subse- 

 quently found to be a mistake. In the case of the two blades, tlie length 

 of the screw was shortened, and consequently the width of the blades 

 reduced, with increased effect, till each was brought down to consider- 

 ably less than one-sixth of the circumference or area of the entire 

 circle; the maximum spied was then obtained. Experiment has also 

 shown that the effective propelling area of the two-bladed screw is 

 tantamount to its entire circle of revolution, and is generally estimated 

 as such. 



Many experiments tried by the author, with various forms of screws 

 applied to a small steam-boat, led to the same conclusion, — that the two 

 blades of one-sixth of the circle gave the best results. 



All screws reacting on a fluid such as water must cause it to yield to 

 some extent. This is technically known as "slip," and whatever the 

 ratio or percentage on the speed of the boat may be it is tantamount 

 to just so much loss of propelling power, this being consumed in giving 

 motion to the w^ater instead of the boat. 



On starting the engine of the steam-boat referred to, and grasping a, 

 mooring rope at the stern, it was an easy matter to hold it back with 

 one hand, though the engine was equal in power to five horses, and the 

 screw making more than five hundred revolutions per minute. The 

 wliole force of the steam was absorbed in "sli])," or in giving motion 

 to the column of water; but let her go and allow the screw to find an 

 abutment on a fresh body of water not liaving received a gradual mo- 

 tion, and with its inertia undisturbed when running under full way, 

 the screw worked almost as if in a solid nut, the "slip'' amounting to 

 onlj' 11 per cent. 



The laws which control the action of inclined surfaces, moving either 

 in straight lines or circles in ^r/r, are identical, and serve to show the 

 inutility of attempting to raise a heavy body in the atmosphere by 

 means of rotating vanes or a screw acting vertically ; foi' uidess the ratio 

 of surface compared to weight is exceedingly extensive, the whole 



