Professor Barlow on the Hydrostatic Pressure,. <$-c. 293 



make water-tight than when it turns on pivots at one edge ; 

 nor does the same aperture pass an equal quantity of water ; 

 for, besides the space occupied by the sluice in the centre, it 

 also tends to disturb the regular flow or current of the water. 

 In all cases, however, where the sluice is large, and the re- 

 servoir deep, there will be a considerable saving in its' con- 

 struction. 



Art. XX. — On the Force exerted by Hydrostatic Pressure in 

 Bramalis Presses ; and on the resisting Power of the Metal, 

 with Rules for computing the thickness of the same for dif- 

 ferent Pressures.* By Peter Barlow, Esq. F. B. S. of 

 the Boyal Military Academy, Woolwich. Communicated 

 by the Author. 



This paper commences by an examination of the amount of 

 the strain exerted in the circumference of the cylinders incon- 

 sequence of any given internal pressure, and the result, al- 

 though somewhat differently obtained, is the same as was first 

 determined by Mariotte, viz. " The circumferential strain, 

 on any given point of the interior of the cylinder, is equal to 

 the pressure of a square inch multiplied by the number of 

 inches in the radius."" That is, the force tending to rend the 

 cylinder along any line parallel to its axis, is equal to the pres- 

 sure on a section between the circumference and axis. This, 

 as we have said, is the result which has always been deduced 

 by writers on this subject ; but in estimating the thickness ne- 

 cessary to resist this strain, it has universally been supposed 

 that all the metal in the thickness opposed an equally resist- 

 ing power ; from which it resulted, that in presses of the 

 same internal diameter, the thickness ought to be proportional 

 to the pressure. This principle, however, is known to fail in 

 practice, it having always been found requisite to increase the 

 thickness in a higher ratio than the pressure, and it was prin- 

 cipally with a view to correct this error, that the author un- 

 dertook the investigation, at the earnest request of some of 

 his practical friends, and having completed it, it has been 



" This Article is an abstract of a paper read bifore the Society of Civir. 

 EsfiiNEras, Feb. 22, 182.5- 



