472 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 795. 



orifice would be the same as through a simple orifice; whereas I found the velo- 

 city to be greater, very nearly in the ratio of \/ 2 to 1, the length of the pipe 

 being equal to the depth of the cylinder. It appears therefore to flow out with 

 about the same velocity as if the pipe had been continued to the orifice. The 

 fluid therefore must have flowed from the orifice in a cylindrical form, for the 

 pipe was observed to be filled. I see no cause which could prevent the vena 

 contracta from being formed. I then stopped the pipe at the bottom i/z, and 

 filled the vessel and pipe, and found the circumstances to be exactly the 

 same. 



In order to determine whether there was any pressure of the fluid against the 

 sides of the pipes as it passed through in all their different situations, I pierced 

 some small holes in them at different parts. In the cylindrical pipes, and those 

 in the form of increasing cones, the fluid passed by tbe holes without being pro- 

 jected out, or without having the least tendency to issue through them; but in 

 the decreasing cones the fluid spouted out at the holes. In the former cases 

 therefore there was no pressure against the sides of the pipes, but in the latter 

 case there was. 



In respect to the motion of the fluid through any of the pipes, I found no 

 difference whether I stopped the pipe at the end of the tube which enters into 

 the vessel, in which case the motion began when the tubes were empty, or whe- 

 ther at the other end, in which case they were full at the commencement of 

 the motion. That the fluid should flow into the top of the pipe faster than it 

 would through an orifice, may probably, in part at least, be owing to the adhe- 

 sion of the fluid to the pipe, and be thus explained. Though the horizontal 

 motion of the fluid towards the orifice accelerates the velocity after it escapes 

 from the vessel by contracting the stream, yet it must diminish the velocity at 

 the orifice; that is, if the same perpendicular motion were to take place without 

 the horizontal motion, the fluid would flow out faster; for as any motion in a 

 fluid is immediately communicated in every direction, the horizontal motion will 

 produce a motion upwards, and in some degree obstruct the descent of the 

 fluid. If therefore this horizontal motion could be taken away, or any 

 how diminished, the fluid would flow out with a greater velocity. Now if a pipe 

 be fixed, the fluid at the bottom of the vessel flowing towards the orifice will, 

 by its adherence to the vessel, continue to adhere to the sides of the pipe as 

 soon as it arrives there, and by this means almost all the horizontal motion will 

 be destroyed, and converted into a perpendicular motion, for the horizontal 

 motion arises principally from the fluid which flows from and very near 

 to the bottom, where the whole motion is very nearly in that direction. 

 This motion therefore being thus nearly destroyed, the fluid will be less 

 interrupted at the orifice, and consecjucntly will flow out with a greater 

 velocity. But why the velocity should also be increased eitlicr by increasing the 



