Mr Watts’ Observations on the Resistance of Fluids. 8S3 
never actually occurs, but which will be more particularly con- 
sidered when we come to reply to the fifth objection. 
5. The only other objection that deserves notice, is, that 
whatever reason can be given for setting out with the principle 
that the pressure on the little surface o, moving with the velo- 
city v^ is equal to niakes it indispensably neces- 
sary to take for the velocity not that with which water would 
issue from a hole, whose depth under the surface is but the 
velocity with which it will issue from a hole whose depth is 
h -p 33 feet ; because this is the velocity with which it would 
rush into the void left by the body, &c. 
This objection is the most plausible of all, and seems, at the 
first glance, to carry considerable weight with it ; but it is not 
very difficult to shew that it is more specious than solid. When, 
indeed, the velocity v is very great, and such that a perfect va- 
cuum is formed behind the small surface o, in this extreme case 
it must be admitted that the objection would be valid; but when 
the velocity v is not very great, the surrounding fluid will press 
in behind the surface o, and, in a great measure at least, balance 
the anterior pressure which it sustains. In like manner, the 
pressure of the incumbent atmosphere, both before and behind 
the surface o, will nearly balance each other’s effects, and be 
transmitted even to the surface o, by means of the intervention 
of the different horizontal strata of the water ; so that when the 
velocity v is not very great, the velocity u.^ with which the water 
would issue from the orifice, at the depth below the upper sur- 
face of the water, will be that which is due to the height h near- 
ly, and not to that of A -f 33, as stated by the objector. 
Thus have I been at some pains to inform the reader of my 
reasons for adopting this theory of the resistance of fluids, not- 
withstanding that it has been asserted by persons, to whose judg- 
ment I pay the utmost deference, to be “ contrary to received 
opinions, and to the most distinct experiments.” 
Custom-House, Penzance, 
May 1820. 
