434 Notes. 



this substance with chloride of silver, &c. In each experiment the 

 branch of the tube containing the mixture was warmed, while the 

 other was cooled with moistened paper. 



IV. 



On the Construction of Valves. 



A VALVE is a kind of lid or cover to a tube or vessel, so contrived 

 as to open one way by the impulse of any fluid against it, and to 

 close, when the motion of the fluid is in the opposite direction, like 

 the clapper of a pair of bellows. In the air-pump this purpose is 

 effected by means of a strip of leather, bladder, or oiled silk, 

 stretched over a small perforation in the piston, and ordinarity in a 

 507. plate at the bottom of the barrel. 



In common water-pumps the valve, or sucker^ as it is often called, 

 is a thick piece of leather pressed down by a small wooden weight, 

 and turning on the flexible leather as a hing^. It is represented at 

 , figure 245, &c. In the best metallic pun ps for raising water the 

 valve consists of a metallic cane with a stem and knob, as represent- 

 ed at L in figure 247. The conical part is ground so as to fit accu- 

 rately the rim of the opening in which it plays, and unlike other 

 valves, it is rendered tighter by use, and is less likely to be ob- 

 structed by foreign substances contained in the water. 



Mr Perkins invented a pump having a square bore, in which the 

 valve consists of two triangular pieces of leather loaded with 

 weights, and turning on a metallic hinge placed diagonally across 

 the bore. Mr Evans adapted a similar kind of valve to the common 

 pump of a circular bore, the form of the valve being a semi-ellipse. 

 The chief advantage of this construction is, that there is very little 

 obstruction to the motion of the water, and consequently less loss of 

 power, than in the common pump, where the space, left for the pas- 

 sage of the water, bears a less proportion to the whole bore. 



