l\<\ MAi ' \M> 



u the fluid to pass through it fn.ni .ne side i.. the nther ig 

 called a //</. 



ipk Piston. The simplest tWin .f piston is a plain 

 cylinder tit tint: accurately another, inside which it moves. 

 'Such a piston works N\ i tli very little fri.-ti.n, hut as thn 

 no adjustment for wear, such a piston is not suitable for a hi^li 

 fluid pressure if it has to work constantly. This simple fn 

 piston is used in the steam-engine indicator, and also in 

 pumps. 



Pig. 56 shows the piston of the circulation pump >t 

 engine. A is the cast-iron casing or Lain -1 <>t tin pump ; B is 

 a brass liner fitting tightly into the former at its ends, ami 

 secured by eight screwed Muntz metal pins C, four at each end ; 

 D is the piston, which is made of brass, and is attached to 

 a Miintz metal piston rod E. Thr liner is bored out smooth 

 and true from end to end, and the piston is turned so as to be 

 a sliding fit to the liner. The wear in this form of pist 

 diminished by making the rubbing surface large. 



EXERCISE 67 : Piston for CirculituKj 1'nmp. Draw the 

 vertical sectional elevation of the piston, &c., shown in ti^. fr. 

 a half plan and half horizontal section through the centre. Scale 4 

 inches to a foot. 



I'n in i> />'"/.'/. The next form of piston which we illustrate 

 is shown in fig. 57. This represents the air-pump bucket of a 

 marine engine. The bucket is made of brass, and is provided 

 with six india rubber disc valves. The rod is in this case 

 made of Muntx. metal. Air pump rods for marine engines are 

 very often made of wrought iron cased with brass. It will he 

 observed that there is a wide groove around the Imrkrt. which 

 is filled with hempen rope or gasket. This gasket t 

 elastic packing which prevents leakage. This is an 

 fashioned form of packing, and is now only used for pump 

 buckets. 



EXERCISE 58 : Air-pump Bucket. Draw the sectional elevation 

 of the air-pump bucket shown in iiu f . :,~. Also draw a half pl;m 

 looking downwards and a half plan looking upwards. Scale In 

 to a foot. 



