ARTIFICIAL REFRIGERATION AND ICE MAKING. LI, 
atmospheres than it was to deal with ether vapour at a very low 
tension, and the result has been along series of inventions having 
for their object the improvement of the compressor. English, 
American, and Continental inventors have all contributed to the 
perfection which refrigerating compressors have now reached, 
most of them keeping one special point in view to the neglect of 
others not so important in their opinion, hence we have a large 
choice of ammonia compressors in the market of most admirable 
workmanship, each one of which is claimed by its respective 
agents to be the best in the world. It is hardly possible that they 
can all be the best seeing the wide divergence which exists in 
their design, proportions, and construction, but it will be instruc- 
tive to examine into the details of the principal machines that are 
made and see how they set about the work they undertake to do. 
It must be admitted that theory and natural laws have no 
favorites, and that the conditions which result from compression 
and expansion are the same for every one. But theory alone is 
of little avail in the work of the mechanical engineer, some of the 
biggest failures have resulted from hugging one main central 
theory so closely that all the little attendent theories were for- 
gotten, and it is the knowledge of the little theories which con- 
‘stitute practical experience, : 
THE CHARACTERISTICS OF AN IDEAL REFRIGERATING MACHINE. 
Refrigerating machines generally consist of a compressor in 
combination with a steam engine, but such compressor may have 
its piston driven by any other power without affecting its efficiency 
for the work of refrigeration. It is therefore desirable in an 
enquiry into the merits of any compressor to consider its various 
Points under separate heads. First the work to be done in com- 
Pressing a gas, for which the design and construction of the com- 
Pressing cylinder with its piston and valves are the principal 
instruments concerned : and secondly the connection of the same 
to the motive power, assuming that any form of engine, slide valve, 
Corliss, simple, or compound, is open for use ; and that therefore 
the merits of the engine itself, apart from its attachment to the 
