PR 



"A-, k {Figs. 1 and 2) is the fra- 

 ming of a forcing pump, with a nar- 

 row barrel ; i is the well containing 

 water to supply the pump. Fig. 3 is 

 F}g. 3. Fig. 4. 



f&^ 



a section of the pump and its valves. 

 The pump, m, is of bronze ; the suc- 

 tion pipe, n, has a conical valve with 

 a long tail ; the solid piston, or plun- 

 ger, p, is smaller than the barrel in 

 which it plays, and passes at its top 

 through a stuffing-box, q ; r is the 

 pressure-valve, s is the safety-valve, 

 which, in Fig. 2, is seen to be loaded 

 with a weighted lever ; t is the dis- 

 charge-valve, for letting the water es- 

 cape, from the cylinder beneath the 

 ram, back into ihe well. See the 

 winding passage in Fig. 4. u is the 

 tube which conveys the water from 

 the pump into the press-cylinder. In 

 Fig. 2 two centres of motion for the 

 pump lever are shown. By shifting 

 the bolt into the centre nearest the 

 pump rod, the mechanical advantage 

 of the workman may be doubled. 

 Two pumps are generally mounted 

 in one frame for one hydraulic press : 

 the larger to give a rapid motion to 

 the ram at the beginning, when the 

 resistance is small ; the smaller to 

 give a slower but more powerful im- 

 pulsion when the resistance is much 

 increased. A pressure of 500 tons 

 may be obtained from a well-made 

 hydraulic press with a ten inch ram, 

 and a two and a one inch set of 

 pumps ■■ — (Ure.) 



PRICKING. The same as nick- 

 ing, which see. 



H H H 2 



PRO 



PRICKING OUT. Thinning 

 plants in drills, &o. 



PRICKLE. A sharp thorn, pro- 

 dnceii by a thickening of the hark or 

 skin of the plant. 



PKI.MJ-: VI.E. The passage 

 through tlie bowels. 



PRIMARY ROCKS. Theunstrati- 

 fied, crystalline rocks, as granites. 

 Sometimes the transition slates and 

 stratitied rocks are mcluded in this 

 term. 



PRIMINE. The outermost cov- 

 ering of the ovule ; afterward it be- 

 comes the skin, or testa, of the seed. 

 P R I .M ROSE. Pretty rtowering 

 perennials of the genus Primula. 



P R I S .M. '■ A solid contained by 

 planes, of which two that are op- 

 posite are equal, similar, and paral- 

 lel, and all the rest parallelograms. 

 Prisms take particular names from 

 the figures of their ends, or opposite, 

 equal, and parallel sides. When tlie 

 j ends are triangles, they are called 

 triangular prisms ; when the ends are 

 square, square prisms ; wlien the 

 ends are pentagonal, pentagonal 

 prisms ; and so on. A nght prism 

 has its sides perpendicular to its 

 I ends ; an oblique prism is that of 

 which the sides are oblique to the 

 ends. The solid conUnt of a prism 

 is found by multiplying the area of 

 the base into the perpendicular alti- 

 tude ; hence all prisms are to one 

 another in the ratio compounded of 

 their bases and altitudes." The opti- 

 cal prism is of three similar sides. 

 i PRISMATIC COLOURS. Light 

 j passing through a prism is divided 

 [ into seven colours, called the pris- 

 I matic speclrum ; these colours are, in 

 ! their order, red, orange, yellow, green, 

 blue, indigo, violet. They have each 

 ' peculiar chemical properties. 



PRH'ET. Ligustrum vulgarc. A 

 small shrub with pretty whhe flow- 

 ers and black berries, used as an or- 

 namental hedging in gardens. They 

 are readily propagated by seeds, lay- 

 ers, or cuttings. 



j PROBANG. CEsophagus tube. 

 j See Ox. The tubes (see Fig.) are of 

 : leather, covering a spring; they are 

 , sometimes hollow, and contain a sti- 



041 



