IQ4 THE SCIENTIFIC PAPERS OF 



not conceive that the whole effect of the machine depended upon 

 the heat (utilized negatively) in that final expansion ; and in 

 order to be sure, as he thought, doling out as it were the com- 

 pressed air to the expanding cylinder, he introduced a throttle- 

 valve into the compressed and cooled air ; and instructions were 

 given to wiredraw that valve as much as possible on the sup- 

 position that after all the safe plan of gaining refrigeration would 

 be to expand air through the throttle-valve, that was through a 

 contracted orifice. Dr. Siemens showed that contraction was 

 the worst thing that could happen that by merely expanding 

 air without exacting work from it, no depression of temperature 

 would ensue. By friction and useless agitation of the fluid, the 

 loss was 3 HP., by condensation of vapour, TOO HP. That, of 

 course, was a fruitful source of loss. If the air was compressed 

 and then not sufficiently refrigerated, the vapour which it con* 

 tained would, on re-expansion and final cooling in the expansive 

 cylinder, not only condense, but be converted into ice. The 

 vapour which went in as steam would come out as ice, and the 

 whole of the latent heat taken up in that double process of con- 

 version was entirely lost. That loss in some of the earlier 

 attempts was fully equal to the whole useful effects that could be 

 expected. But the greatest loss of all was incurred in allowing 

 the cold air to leave the apparatus at the minimum temperature. 

 Dr. Siemens suggested that a current exchanger should be supplied ; 

 that the cold air, after it had done its negative work, so to speak, 

 should pass up through spaces surrounding a number of vertical 

 tubes, and that the compressed air should pass through the tubes 

 themselves in the opposite direction, and exchange temperatures. 

 In that way 1\ HP. could be saved. It was shown in the report 

 that a very economical result might be obtained by observing 

 these precautions. Attached to the report were diagrams taken 

 from the three cylinders, and a sketch showing the kind of 

 apparatus which was then proposed, and which, he thought, in all 

 material points was similar to the machine brought before the 

 members by the author of the paper. There was, however, one 

 point in which he still dissented from the knowledge and experience 

 of the present day. He had stated in the report that as ice-making 

 was really negative work, there could be no absolute limit to the 

 effect to be produced by heat in that direction. It was true that 





