L. MECHANICS AND ENGINEERING. 507 



to the public; but there are several difficulties connected 

 therewith, the principal one being its bulkiness, as large 

 vessels must be used in order to get a sufficient amount of 

 motive power, and torpedoes can only be driven in this man- 

 ner for short distances, the air being used at a pressure of 

 from 600 to 900 pounds to the square inch. The use of 

 liquefied gas has the advantage that a much larger quantity 

 of motive power can be stored in the same space ; of the gases 

 that may be liquefied, carbonic acid is the best. In the em- 

 ployment of this substance as a motive power for torpedoes, 

 it becomes necessary to attach to the torpedo a reservoir 

 containing the liquefied gas, under a great pressure, the es- 

 cape of which propels the torpedo through the water. The 

 liquefaction of the gas- may be accomplished either by the 

 aid of the pressure of the evolved gas or by means of me- 

 chanical compression. The former method has been very 

 generally employed in scientific experiments on a small 

 scale. Thilorier's apparatus for this purpose has the advan- 

 tage of simplicity, but is troublesome and wasteful of the 

 gas. By the mechanical processes, all the gas generated 

 may be condensed, and at less expense, although the first 

 cost of the apparatus is considerable. The Lay apparatus 

 used at the Newport Torpedo Station is based on the meth- 

 od of condensation by the pressure of the gas itself; and the 

 experience with this confirms the previous opinion that con- 

 densation by the use of compressing pumps is much better. 



After the apparatus provided with the Lay torpedo had 

 become unserviceable for want of repair, a second one was 

 prepared from specifications made by Mr. Hill, in which com- 

 pressing pumps were employed. The pump used in this ap- 

 paratus was designed and built by the Burleigh Rock Drill 

 Company, and is a modification of their well-known air com- 

 pressor. The compressor and the generator were finished in 

 April, 1874, and are fully described in Mr. Hill's pamphlet. 

 The novel and most important feature of the apparatus is the 

 supplying the gas to the compressing pump at a high generat- 

 ing pressure. The pump has then only to compress this gas 

 to a moderate extent to bring it to the liquefying point. One 

 hundred pounds to the. square inch is the average pressure 

 of the delivered gas. Suppose now the pressure of liquefac- 

 tion to be 600 pounds. Then the gas must be compressed 



