MECHANICS AND USEFUL ARTS. 25 



stop-cocks on the tubes be opened, and the pressure of the steam 

 increased, the air in the gauge \vill be compressed proportionally, and 

 the water will rise to an equal height in each branch of the tube ; in 

 this way the gauge may be graduated by direct experiment. But the 

 fall of the water level in the boiler will cause the level to fall also in 

 that branch of the gauge which communicates with the lower tube, 

 (that is, the tube opening near the bottom of the water space of the 

 boiler,) and this will cause the water to rise hi the opposite branch of 

 the gauge, hi consequence of the necessity of the column of air retain- 

 ing its bulk. While, therefore, the pressure of steam in the boiler is 

 indicated by the mean height of the columns in the gauge, the fall of 

 the water below its adjusted level will be indicated by the difference of 

 the height of these two columns, provided the level of the water in the 

 boiler end of the upper tube be maintained constant. 



In practice, this construction is modified by the introduction of another 

 vertical tube, connecting the end of the upper and lower tubes near the 

 boiler. The upper tube is then inserted into the steam space of the 

 boiler, and it leaves the connecting tube at the proper water level, 

 when it runs as before described ; in this way there will be left but a 

 small portion of the upper tube to be filled by the condensed steam. 

 The lower tube is also provided with a blow-off cock between the boiler 

 and the stop-cock before described to prevent this tube from being 

 choked by sediment. The level of the water in the gauge is indicated 

 by a floating glass tube, colored and graduated in the inside, and closed 

 in the leg communicating with the upper tube, while a glass ball floats 

 on the surface in the other leg. The difference in the levels of the 

 water columns is then indicated by the position of this ball on the grad- 

 uated scale of the glass tube in the other leg. 



NASMYTH'S ABSOLUTE SAFETY VALVE. 



THE chief feature which distinguishes this improved safety valve from 

 all others hitherto proposed, consists in the peculiar and simple manner 

 in which the motion of the water in the boiler is employed as an agent 

 by which the valve is prevented from ever getting set fast in its seat. 

 The swaying to and fro sort of motion, which, at all times, accompanies 

 the ebullition of water in boilers, is made to act upon a sheet-iron 

 appendage, attached to a weight, which weight is connected directly 

 with a brass valve ; and as the rod which connects this sheet-iron 

 appendage and weight to the valve is inflexible, it will be easily seen 

 how any slight pendulous motion given to it is directly transferred to 

 the valve ; and as that portion of the valve which rests hi the seat is 

 spherical, the valve not only admits of, but receives, a continual slight 

 motion in its seat, in all directions, as the result of the universal pendu- 

 lous motion of the appended weight, as acted upon by the incessant 

 swaying motion of the water during ebullition. 



IMPROVED LOCOMOTIVE. 



AT the Great Exhibition, a locomotive was exhibited by Messrs- 

 Hawthorn, for which they claimed a capability of running, with safety, 



o 



