AERIAL LOCOMOTION 429 



the enemy's force of harbor defenses, and by keeping the machine 

 circling above a battleship or a fleet of ships, the possibility of attack 

 by submarine boats would be very greatly lessened. In fact I should 

 think that with Dr. Bell's multicellular machine there would be no great 

 difficulty in maintaining the operator in the air for hours by simply 

 flying the machine as a kite anchored to the ship. 



I trust that Dr. Bell will pardon me for not agreeing with the 

 explanation he suggested of the very interesting fact noted in regard to 

 the propulsion of the " Catamaran Life Raft " by means of aerial pro- 

 pellers, namely that the raft advanced against a 16-mile breeze, 

 although in a calm it was able to make only something like four miles 

 an hour. 



It seems to me that this ability of the raft to advance against a 16-mile 

 wind is not due to the difference between the momentum of the raft 

 and the momentum of the air, but to the fact that the raft presents 

 very little resistance to the wind, while the propeller, being revolved 

 at a high rate of speed by the engine, tends to advance in the air at a 

 speed proportionate to its pitch multiplied by its number of revolutions 

 in a given time ; and I have no doubt that the raft would have advanced 

 against any wind of a velocity less than that which would be created 

 by the slip of the propeller revolving in still air at the same speed as 

 when driving the raft. In other words, if the propeller had a pitch, 

 let us suppose, of one foot (that is, tended to advance through the air 

 one foot for each revolution, or forced the air backwards one foot for 

 each revolution), such a propeller revolving at the rate of a thousand 

 revolutions a minute would in a calm create a back wind of a thousand 

 feet per minute, and of course a propeller of two feet pitch would 

 create a back wind of two thousand feet per minute when revolving at 

 the same speed. Such a propeller, then, of two feet pitch, revolving 

 at this speed, when mounted on a raft should be able to prevent the 

 raft being blown backwards in a wind of somewhere near two thousand 

 feet per minute. I have no doubt that the back wind due to the pro- 

 peller in Dr. Bell's experiment was of an even higher velocity than 

 two thousand feet per minute. 



Few of us can conceive of the affairs of the world being very differ- 

 ent from what we are accustomed to. But there are certain definite 

 effects which we can be fairly confident will follow definite changes. 

 I am not a prophet nor the son of a prophet, but I feel safe in ventur- 

 ing a conservative prediction in regard to one of the effects of aero- 

 dromic work in the next few years. We may not be able to make it 

 a general vehicle of transportation, as some enthusiasts predict; I my- 



