MECHANICS AND USEFUL ARTS. 27 



copper box will rapidly increase. This increase will be indicated by 

 the water-gage, and notice will be given before the extra heat be- 

 comes dangerous. The safety-valve can then be raised, so as to let 

 off some of the steam, which will cause the water to foam up and cool 

 the surface of the flues, and if the alarm is attended to in time there 

 will be no danger in this. But if it is neglected, the only safe way is 

 to partially extinguish the fires and start the pumps. 



The steam-gages are so constructed that, if any more air is intro- 

 duced than there was before the scale was made, the extra quantity 

 will escape the first time the steam is down ; and the scales cannot be 

 slid upwards so as to make them indicate less than the true pressure, 

 for this would be at once detected by comparison with the stuffing- 

 boxes that hold the glass tubes. The only methods of interfer- 

 ing with the correctness of the water-gage would be by cutting a 

 hole in the box on the flue, thus letting the water out into the boiler, 

 or by fastening down the alarm-valves. If the former were done, the 

 vibrations of mercury in the two gages would correspond, and so in- 

 dicate that something was wrong. If the valve were fastened down, 

 the iron tube would burst open, and this must take place before the 

 flues were heated to 450, and as it requires 1,000 to produce a red 

 heat, timely warning of this would be given. There are several other 

 guards against all possible derangements of this very ingenious inven- 

 tion, which is the best preventive of explosions we have yet seen. 

 Scientific American. 



NEW ROTARY ENGINE. 



SEVERAL trials have been made, as we learn from the London Times, 

 with a rotary engine, which has been brought to its present working 

 condition by Hon. W. E. Fitzmaurice. The engine is very simple, 

 merely consisting of two pieces so mathematically arranged that the 

 interior part works in the outer with the greatest ease, being free from 

 dead points and without the slightest vibration, however great the ve- 

 locity. It has no springs or packing, and the parts meet each other so 

 harmoniously as only to give a humming noise like a spinning top, "and 

 it is not in the least liable to get out of order, the wear being per- 

 fectly uniform throughout. The entire motion being a rolling in- 

 stead of a cutting one, the engine will last long without repair, as 

 the surface becomes case-hardened in a very short space of time. 

 The trials took place in the presence of several scientific gentle- 

 men and engineers of eminence in their profession, in a frigate's 

 pinnace, the engine being constructed for the government. The boat 

 is of 10 tons burden, carrying a load of 5- tons, and drawing 4 feet of 

 water. She is 32 feet long and 8 feet breadth of beam, made for car- 

 rying men and carronades, but not in any way calculated for speed, 

 and yet the engine of 10 horse-power, occupying a space of 21 inches 

 by 7 inches, drove a screw-propeller of 3 feet in diameter and 4 feet 

 pitch with such velocity as to make 200 revolutions in a minute, the 

 motion being given on the direct-action principle. Although the 

 boat was not at all calculated fur speed, she was propelled against the 



