Polytechnic Association. 75 j 



Mr. T. Shaw— With one propeller there is a maximum height 

 which you can lift the water. 



Prof. R. H. Thurston — It seems tome that, theoretically, Mr. Shaw 

 is right. Starting at the bottom, with the water at rest, rotating tine 

 screw, it meets the water below this first guide curve or stationary 

 blade. If that were not there, it would give the water a motion at" 

 revolution; and as soon as that becomes equal to the motion of the 

 screw, we have reached the maximum elevation possible with that 

 screw. But this guide curve takes the water, and from its rotary 

 motion it rises directly upward, to a point where the pressure due to 

 the height prevents its rising farther. At this point there is placed 

 a second propeller, which finds the water quiescent, and repeats the 

 operation, lifting the water again to this point, when the third pro- 

 peller takes it, and so on. Now, suppose we begin again, with the 

 blades brought nearer together. The water rises above the first guide 

 curve with a pressure sufficient to take it up six feet ; that is, with a 

 pressure of three pounds to the square inch. The second propeller 

 finds the water rising directly upward, and adds its pressure of three 

 pounds to the square inch, so that the water rises above the second 

 guide curve with a pressure of six pounds to the square inch. Still 

 rising directly upward, the third propeller adds its force, making the 

 pressure nine pounds, and so on. This view of the theory is corrobo- 

 rated by the statement of Mr. Shaw, that that is the actual fact. 



Dr. Van der Weyde — That would be true if the water were not in 

 motion ; but the second propeller finds it in motion, and cannot add 

 to that motion. The second propeller must be placed where the 

 ascending motion has reached its highest point, and then it will carry 

 it still further. 



Mr. Dudley Blanchard — I think the effect would be the same as if 

 we were to station several men on a ladder to pass up bricks from 

 one to another. A dozen men placed far enough apart could toss a 

 brick up to a considerable height. But place them together near the 

 bottom, the lower ones throw it up a foot, the last one as far as he can, 

 and that is as high as it will go. 



Mr. T. Shaw — I do not think that is a parallel case. 



Mr. T. D. Stetson — Let us consider for a few moments the ques- 

 tion of friction in this pump. The friction which Dr. Van der 

 Weyde measured by the heating of the water escaping from the 

 water-bearing, was only one element of friction in the machine. That 

 was only the friction from the end pressure, whether at the bottom or 

 the top, in supporting the pipe and its contents. In addition to that, 



