TRANSACTIONS OF THE SECTIONS. 211 



On Giffard's Injector for Feeding Boilers. By William Froude. 



In this instrument a jet of steam taken from the boiler and issuing from a pro- 

 perly tapered orifice, is met by and enveloped in a regulated supply of water, either 

 cold or of limited temperature. 



The column formed by the combination of water and steam is made to impinge 

 on the aperture of a similarly tapered orifice, of rather smaller area, connected with 

 the feed-pipe ; and penetrating this orifice, it flows in a continuous stream into the 

 boiler. 



The rationale seems to be as follows : — were it possible to condense such a jet of 

 steam by a simple abstraction of temperature, it would collapse into a jet of water 

 having only y^ o tn of its previous sectional area, its particles, however, retaining 

 the same weight and velocity, and therefore the same momentum for each unit of 

 time which they had possessed as steam. And since the momentum of a jet is the 

 exact dynamic equivalent of the pressure which produces it, this water-jet would 

 possess a momentum equal to that of a jet of equal diameter taken from a boiler 

 having 1700 times the pressure of that from which itself had issued as steam, and 

 would be capable of penetrating a boiler having a pressure enlarged almost in the 

 same proportion. 



In the injector the water which is added condenses the steam and becomes incor- 

 porated with it, forming a compound jet which possesses for each unit of time the 

 same momentum which the jet of steam possessed. And if tbe supply of water be 

 duly regulated, the sectional area of the compound jet may be precisely adapted to 

 the orifice of the feed-pipe. 



Now were that orifice equal in area to the steam-jet orifice, and were a jet of 

 water allowed to issue from it under the same pressure which discharged the steam, 

 the water-jet would have the same momentum for each unit of time as the 

 steam-jet had, and therefore as the compound jet derived from it ; and the two 

 would precisely neutralize one another when brought into opposition. If, however, 

 the steam-jet orifice be the larger of the two, then the jet derived from it, if reduced 

 by condensation to the diameter of the smaller, will be the stronger in the same 

 proportion, since it will possess the momentum due to the larger area of pressure ; 

 it will therefore drive back the water which is striving to escape from the feed-pipe, 

 and will pass as a continuous stream into the boiler. 



The water supply is considered to be correctly adjusted when the passage takes 

 place without an overflow of steam or water ; but the test is deceptive ; for an over- 

 flow of steam merely implies that the supply of water is barely sufficient to condense 

 the steam into a jet as small in section as the feed-pipe orifice ; an overflow of water 

 merely implies that though the steam is fully condensed, the supply of water has 

 enlarged the compound jet to a section exceeding that of the feed-pipe orifice. 



In reality the operation should be brought as near as possible to the latter limit ; 

 for though it will indeed seem to be proceeding quietly and properly in all the inter- 

 mediate stages, it will be found that the compound jet, when not so enlarged as to 

 fill the feed-pipe orifice, possessing its full momentum in a smaller section, will have 

 energy enough to take up with it and carry into the boiler a considerable quantity of 

 air, wasting thus not only its own power, but in a high degree that of the engine 

 also, when it is a condensing one, since it encumbers the air-pump with extra duty. 



On a Process for covering Submarine Wires with India-rubber 

 for Telegraphic purposes. By Walter Hall. 

 The author exhibited a model of his machine, which effected the object by wind- 

 ing- strips of rubber, and moistening- the same with naphtha during the process of 

 covering ; the wire thus formed being covered with a thread of vulcanized India- 

 rubber, and the whole afterwards subjected to a temperature of 140°. The wires 

 thus covered were protected with a plaited covering of hempen cord, into which 

 longitudinal steel wires were introduced for the purpose of giving strength. 



Suggestions relative to Inland Navigation. 

 By Professor Hennessy, F.R.S. 

 The fact that the forces operating in canal and river navigation are so different 



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