FORM OF A SPONGE 307 



we may expect nearly twice the oscular velocity, and conse- 

 quently nearly twice the diameter of supply, with this 

 admittedly more specialized type of canal system. 



This pressure of 1 mm. water is transmitted, with a loss 

 from friction, through the efferent canals and cloaca to 

 the open osculum, where the potential energy of the com- 

 pressed water is converted into the kinetic energy of the swift 

 oscular jet. 



We are familiar with such conversion of the potential energy 

 of compressed air in an air-gun into the kinetic energy of the 

 escaping bullet, and of the potential energy of the compressed 

 water at the bottom of a cistern, converted into the kinetic 

 energy of the jet from a garden-hose. We know well, with the 

 garden-hose, that when the water will only go 3 ft. from the 

 open hose, it will throw a jet 30 ft. from a fine tube-nozzle. 

 This is because with the open pipe, delivering perhaps 10 gallons 

 a minute, there is a flow of 5 ft. a second through the 1 in. 

 hose-pipe, with great loss of energy in every foot of the pipe 

 from friction. Putting on a nozzle which will only deliver 

 1 gallon a minute, the velocity within the hose is lowered to 

 6 in. a second, -lo of the loss by friction is avoided, and 

 the potential energy of the cistern is transmitted almost 

 undiminished through the hose, in any part of which there is 

 nearly the full pressure of the cistern. If we stop the nozzle 

 with the finger, we have the full pressure of the cistern through- 

 out the hose, and can feel it on the finger. This is not per- 

 ceptibly diminished by allowing a fine thread of water to 

 escape, and its velocity of issue approximates to the full 

 theoretic velocity due to the cistern head, but it does not travel 

 far owing to the smallness of its mass compared with the surface 

 of friction it exposes to the air. As the jet is allowed to increase 

 in volume, so does the velocity increase in the hose-pipe, and 

 the consequent successive loss of energy in each yard of its 

 length ; and we can feel the pressure on the finger noticeably 

 diminish and can see that the velocity of the jet consequently 

 decreases, though with its greater volume it travels farther. 



So with the sponge. The narrow osculum, 2 mm. diameter, in 



