248 



Hydraulic Ram. 



Vol. XI. 



rope hanging over a pulley; and certainly 

 H. H. S. will not contend that it requires a 

 force at A equal to the weight of the rope 

 B C D, to prevent the rope running over, 

 but only a power equal to the weight of C D, 

 A and C being on a level. 



V^y 



But the whole matter may be reduced to 

 experiment, — if we place the finger on the 

 end of a syphon, in order to stop it, it will 

 be found that it is by no means necessary to 

 apply a force equal to the weight of a col- 

 umn of 32 feet in perpendicular height. It 

 is sufficient to apply a power equivalent to 

 the weight of a column whose length is the 

 difference of the perpendicular heights of 

 the two legs. 



Inasmuch then as no contrivance can in- 

 crease the power of the fall, it might be 

 supposed that the ram would work equally 

 well, whether placed close to the spring, 

 or at some distance from it: this however 

 would be a hasty conclusion. A certain 

 time, depending on the structure and size of 

 the ram, is requisite for the perfect opening 

 and closing of the valves ; and to give that 

 time, a long pipe is necessary. Could the 

 valves be opened and closed instantaneously, 

 it would be best for the pipe to be as short 

 as possible ; the diminished discharge from 

 each stroke being more than counterbal- 

 anced by their increased number. But time 

 is necessary; and as we must get it, it is 

 best of course to do so in the most economi- 

 cal manner possible. 



A certain velocity is necessary to close 

 the escape valve. Now the whole moving 

 force being that due to the perpendicular 

 fall, the time necessary to acquire that velo- 

 city will depend on the length of the supply 

 pipe, and be exactly proportional to that 

 length; no allowance being made for fric- 

 tion. Consequently with a supply pipe 40 

 feet long, the strokes will occur about half 

 as often, as when the pipe is 20 feet. But 

 the column being twice as heavy, each 

 stroke will discharge twice as much water; 

 it being assumed that, in both cases, the 

 valves have full time to act. The moment- 

 um and the number of strokes depending 



upon the length of the column; the height 

 being supposed invariable, it will manifestly 

 be best to use a straight pipe, because it is a 

 well ascertained fact, that all curvatures di- 

 minish the discharge. If then the above 

 principles be correct, the effect of H. H. S.'s 

 machine would be increased by straighten- 

 ing his pipe and removing his ram to a dis- 

 tance from the spring equal to the length of 

 his syphon. 



It may be asked if it will make no differ- 

 ence whether the strokes take place at long 

 or short intervals. It undoubtedly does. 



The escape valve is closed by the water 

 pressing against its lower surface. This 

 action depends solely on the velocity of the 

 discharge. It can therefore make no differ- 

 ence in the time occupied by a stroke, whe- 

 ther this velocity is suddenly or more slowly 

 acquired. Suppose we have two precisely 

 similar machines worked by springs havino- 

 the same fall, the supply pipes being re- 

 spectively 40 and 60 feet long. Now in 

 these cases the strokes will be made in the 

 same time, while the intervals between 

 them will in one case be twice as great as 

 in the other. If then an interval of two 

 seconds be required by the ram supplied by 

 the shorter pipe, it will evidently require 

 four seconds to acquire the necessary velo- 

 city in the other; therefore, if half a second 

 is occupied in closing and again opening the 

 valve, it is evident in one case the strokes 

 will be made every two and a half seconds, 

 and in the other every four and a half se- 

 conds ; the number of strokes are therefore 

 as nine to five, and consequently the ma- 

 chine with the longer pipe will discharge 

 the most. No accotmt has been made of the 

 effect of friction, which, if the pipe is ex- 

 tended much will so far diminish the dis- 

 charge, as greatly to lessen the efficacy of 

 the machine. The most economical length 

 will depend upon the size of the supply 

 pipe. From 40 to 50 feet is thought to be 

 the best length for pipes of two inches bore. 

 There is one other remark in the commu- 

 nication of H. H. S., which, as it may possi- 

 bly lead to unnecessary expense in procuring 

 large air vessels, requires a passing remark. 

 He says, "The forcing power of the air ves- 

 sel, like the power of a Bramah press, will 

 increase in proportion to the enlargement of 

 its horizontal area." He certainly could not 

 have considered that it is only the pressure 

 per square inch, that is to be taken into con- 

 sideration; a pressure of 2000 pounds on a 

 surface of 144 square inches, would produce 

 no greater discharge than one of 500 lbs. on 

 a surface of 36 square inches. 



S. Alsop. 



Wilniingion, Del,, Third mo. 1st, 1847. 



