MECHANICS AND USEFUL ARTS. 29 



steam-engine is shared by M. Lenoir's, viz., all the heat generated cann .' . 

 be converted into power. If there was nothing to hinder the complete 

 expansion of the gases, the temperature of the expanded gas would be as 

 lo\v as before the combustion; but, after a certain point of dilatation is 

 reached, the expansive force left is not sufficient to move the piston, and the 

 air must then be turned into the waste-pipe, though still very highly heated. 

 On the other hand, there are several advantages claimed. Besides the low 

 prime cost and alleged economy of fuel, there is a great saving from the 

 facility of staiting the machine in an instant certainly a very great advan- 

 tage, considering the loss of time and fuel attendant upon raising steam. 

 Then there is considerable expense involved in stopping a steam-engine, 

 which is obviated here; the combustion in Lenoir's engine being stopped 

 instantaneously by the turning of a button. 



ON A METHOD OF TESTING THE STRENGTH OF STEAM BOILERS. 



BY DR. JOULE. 



The author adverted to the means hitherto adopted for testing boilers. 

 First, That by steam pressure, which gives no certain indication whether 

 strain has not taken place under its influence, so that a boiler so tested may 

 subsequently explode when worked at the same or even a somewhat less 

 degree of pressure. He trusted that this highly reprehensible practice had 

 been wholly abandoned. Second, That by hydraulic pressure obtained by a 

 force-pump, which does not afford an absolutely reliable proof that the boiler 

 has passed the ordeal without injury, and, moreover, requires a special appa- 

 ratus. The plan which had been adopted by the author for two years past, 

 with perfect success, was free from the objections which applied to the 

 above, and is as follows : The boiler is entirely filled with water, then a 

 brisk fire is made in or under it. When the water has thereby been warmed 

 a little, say to 70 or 90 Fahrenheit, the safety-valve is loaded to the pres- 

 sure up to which the boiler is to be tested. Bourdon's or other pressure 

 indicator is then constantly observed; and if the pressure occasioned by the 

 expansion of the water increases continuously up to the testing pressure, 

 without sudden stoppage or diminution, it may be safely inferred that the 

 boiler has stood it without strain or incipient rupture. 



In the trials made by the author, the pressure rose from zero to sixty-two 

 pounds on the square inch in five minutes. The facility of proving a boiler 

 by this method was so great, that he trusted that owners would be induced 

 to make those periodical tests, without which fatal experience had shown that 

 no boiler should be trusted. Newton's (London) Journal. 



THE GAUGE OF RAILWAYS. 



The London Engineer says that experience has demonstrated a narrow 

 gauge to be decidedly superior to a broad gauge for railways. The power 

 required to work them is much less, broad gauge roads requiring engines 

 and carriages of excessive weight. The broad gauge necessitates longer 

 axles, which increase the liability of one wheel to run ahead of the other 

 on the same axle, to which there is a tendency on all roads, and a conse- 

 quent binding of the wheels between the rails. It is perfectly established 

 that the narrow gauge affords sufficient space for the heaviest engines. 



3* 



