406 WRITINGS OF JOSEPH HENRY. [1874 



The siren. — Of the three machines this is the most complicated. It uses 

 steam at a high pressure, and some of its parts move with very great velocity, 

 the siren spindle making from 1,800 to 2,400 revolutions per minute. The 

 boiler must be driven to its full capacity in order to furnish sufficient steam. 

 A large quantity of steam is at intervals suddenly drawn from the boiler, 

 causing a tendency to foam and to eject a considerable amount of water 

 through the trumpet. 



The constant attention of the keeper is required to regulate the fire, the 

 supply of water to the boiler, of oil to the journals, &c. 



In general terms, it may be stated that the siren requires more skill and 

 attention in its management than either of the other signals. 



The Daboll trumpet. — As the caloric engine, which has been hitherto era- 

 ployed to operate this signal, requires little fuel, no water, and is perfectly 

 safe as regards danger from explosion, it would — at the first glance, appear 

 to be the most suitable power that could be applied to fog-signals, and was 

 accordingly at first exclusively adopted for this purpose. It was however 

 found to be so liable to accident, and so difficult to repair, that of late years 

 it has been almost entirely rejected. In the steam-boiler, the furnace is sur- 

 rounded by water, and it is impossible under ordinary circumstances to 

 heat the metal much above the temperature of the water. The furnace of 

 the caloric engine is surrounded by air, and is therefore liable to be burned 

 out if the fire is not properly regulated. 



The working-piston is packed with leather, and as it moves horizontally, 

 with its whole weight resting on the lower side of the cylinder, the packing 

 at its lower edge is soon worn out. 



If the engine is allowed to stop with the piston at the furnace-end of the 

 cylinder the leather is destroyed by the heat. The re-packing of a piston is 

 a difficult and expensive operation, requiring more skill than can be expected 

 among the class of men from which light-house keepers are appointed. 



Another accident to which these engines are subject arises from a sudden 

 check in the velocity of the piston, caused either by the jamming of the 

 leather packing or the introduction of dirt into the open end of the cylinder, 

 in which case the momentum of the heavy eccentrically-loaded fiy-wheel is 

 almost sure to break the main rocker-shaft. 



The expense of repairs is considerably increased by the fact that these 

 engines are not now in general use, and when important repairs are required 

 it is usually necessary to send to the manufacturer. 



This signal requires much attention. The fires must be carefully regulated 

 to avoid burning out the furnace, the journals thoroughly oiled, and the 

 cylinders well supplied with tallow. 



The steam-whistle. — This machine requiring much less steam than the 

 siren in proportion to the size of its boiler, there is not the same necessity 

 for forcing the fire ; the pressure of steam required is less, and the point from 

 which it is drawn much higher above the water-level in the boiler, and there 

 is consequently no tendency to foam. 



The machinery is simple; the piston pressure very light, producing but 

 little strain on the different parts of the engine, which is therefore not liable 

 to get out of order and requires no more attention than a common stationary 

 engine. 



One marked advantage possessed by this signal is that should the engine 

 become disabled, the whistle may still be sounded by working the valve by 

 hand. This is not the case with the two others, where an accident to any 

 part of the machinery renders the signal for the time useless. 



It will thus be seen that the siren is the most expensive of the fog-signals 

 as regards maintenance, and that it is adapted only to such stations as are 

 abundantly supplied with water and situated in the vicinity of machine- 

 shops where the necessary repairs can be promptly made. 



