MECHANICAL AND USEFUL ARTS. 17 



COMBINED STEAM. 



, There has been read to the Institution of Civil Engineers a paper 

 "On Combined Steam," by the Hon. John Wethered, U. S. The 

 author remarked that, at the present day, the great desideratum in 

 marine engines appeared to be, to obtain increased power, or 

 economy in the consumption of fuel, without the commercial dis- 

 advantage of occupying more space, by the enlargement of the 

 boilers and machinery. This object, it was believed, had been at- 

 tained by the application of ordinary and superheated steam mixed. 

 The mode adopted in carrying out tliis system was to attach another 

 steam pipe to the boiler, for conveying the steam to be superheated 

 to pipes, or other contrivances, placed in any convenient form near 

 the fire, or in the uptake, or chimney of the boiler, or in a separate 

 furnace ; the superheated steam being added to the ordinary steam 

 at, or before its entrance into, the cylinder. In its passage through 

 the superheating apparatus, that portion of the steam was raised, by 

 the waste heat, to a temperature of 500° or 000° Fahrenheit. The 

 heat, thus arrested, was conveyed to and utilized in the cylinder by 

 its action on the other portion of steam from the boiler, which was 

 more or less saturated, according to circumstances. The combined 

 steam was used in the cylinder at from 300° to 450° Fahrenheit, 

 instead of at the low temperature at which steam was generally em- 

 ployed. The effect of using the two kinds of steam was, that the super- 

 heated steam yielded a portion of its excess of temperature to the 

 ordinary steam, converting the vesicular water, which it always con- 

 tained, into steam, and expanding it several hundredfold ; whilst, at 

 the same time, the ordinary steam yielded a portion of its excess of 

 moisture, converting the steam gas into a highly rarefied elastic 

 vapour — in other words, into pure steam at a high temperature. 



When steam was merely superheated or dried, it was converted 

 into steam gas. It consequently partook of the nature of gas, was a 

 bad conductor of heat, and gave out with difficulty the heat neces- 

 sary to transform it into mechanical power. On the other hand, 

 mixed steam participated in the qualities of steam proper and of 

 superheated steam, and being a pure, highly rarefied vapour, which 

 readily parted with its heat, thus produced greater mechanical 

 effect. 



By the application of combined steam the following advantages, 

 among others, were said to be obtained : — 1°. An economy of fuel of 

 from oO to 50 per cent. 2°. A diminution of one-third in the feed 

 water. 3°. The employment of smaller boilers to produce the same 

 power. 4°. Facility of maintaining any desired pressure, or of in- 

 creasing it at will incases of emergency. 5°. A steamer would make 

 a voyage one-third further with the same weight of coals, or one- 

 third the space now occupied by the fuel might be used for freight. 

 6°. Less risk of explosion. 7°. Boilers would last one-third longer. 

 8°. A better vacuum was obtained. And 9°. One-third less injection 

 water was required. 



The discussion upon this subject occupied the whole of the next 

 evening's meeting. At the close, it was stated that the general opinion 



B 



