\, Moving Force.) (239 
:, Now. that, velocity cannot) be determined by 
_, measuring the imperfectly contracted vein in 
eases, of water apesttings none a hole in a 
. thin Phlates:(: one 
We may safely ‘afeed infos ‘tet in euch 
| cases, the velocity. is, considerably, less, than 
what i is due to the, head. For, the jet being 
| opaque, some moving force must be expended 
in . Separating the particles from each other, 
and the distance to which the jet from such an 
orifice i is projected on a horizontal plane, con- 
firms. that. inference, The, demonstration, 
therefore, of, the. reaction can be properly ap- 
plied . to such. cases only as, those, where the 
_ water, issuing through a tube properly, con- 
tracted, acquires the velocity nearly which . 
is. due to the head, and in. those cases the 
experimental results agree, as I have stated, 
remarkably well with the demonstration. a 
These results agree also with the explana- 
tions which have been given of moving force. 
If we suppose the velocity of the jet to, be 
equal to that which is due. to the -head,; and 
the vessel to move uniformly in the opposite 
direction cD with the same, velocity ;. the 
water will be at rest asitissues: ., ©, |.) 
Let a represent. the area of, the. ieaaliies 
: ‘section. of the orifice. "Then ; while the vessel 
has moyed ‘through a space =2 BC, a quantity 
