LIl. NORMAN SELFE. 
compressing apparatus, need not take up our time; but it is 
important that the design of the whole machine should be such as 
to simplify its erection and secure economy of power in its subse- 
quent working. Thirdly, there is the provision for easy examin- 
ation of parts, maintenance in repair and working order, which 
should be full and ample. 
Under the first head may be placed the following characteristics 
of a compressing cylinder which are directly concerned with the 
work done on the gas :— 
1. On the in or suction stroke the cylinder should fill with gas 
at a pressure as little below that in the expansion coils as possible 
and the outlet valve should be “ tight.” 
2. The piston and rod should work with the maximum of 
“tightness” in order to prevent leakage ; and with the minimum 
of friction so as not to generate heat or require extra power to 
move them. 
3. On the out stroke the inlet valve should not permit any 
leakage, and the whole contents of the cylinder, less the minimum 
of clearance, should be discharged through the outlet valve at a 
pressure as little above that in the condenser as possible. 
Under the second head, dealing with the general design and 
construction of the whole compressor :— 
4. The machine, other things being equal, should be self con- 
tained on one sole plate so as to be easily erected at the minimum 
of expense for foundations ; because the cost of the foundations— 
and very costly ones are required by some compressors and their 
steam engines—must be taken into account before it is possible 
to make a proper comparison of the cost of different machines of 
the same power in working order. 
5. If it is required to minimise the strain on the crank pins, 
shafts, and connecting rod, and keep down the weight, cost, frie- 
tion, and wear of those parts; the work of the compressor, crank, 
rods, and crossheads should be double acting instead of single 
acting, and the ratio of compression should be as small as possible 
