The mode of setting this boiler is also of considerable importance, as it is advisable to give a long and waving course to the chimney.
A A still represents the principal boiler, while the figures 1, 2, 3, &c. indicate the passage of the flame and heated air; a section of the chimney being shewn at O.
The steel-yard safety-valve which was employed in all the early engines is simple, and the nature of its construction may readily be understood. A represents a portion of the upper part of the boiler; B the safety-valve or plug made to fit air-tight on the valve-seat beneath; C the lever working on its axis at D, and furnished with a moveable weight E, adjusted to balance the pressure of steam within the boiler.
When steam of great elasticity is required, the weight is placed at the extremity of the lever, and as such, acts with greater force on the safety-valve, than when removed to a point nearer to the axis on which it revolves: so that should low pressure steam, or that which has a less expansive force, be required, it will only be necessary to remove it nearer towards the axis on which it turns.
The lever and balance-ball safety valve already described, appear but little calculated for those engines in which high pressure steam is employed, as the engine-man, in an over anxious zeal for the full performance of the machinery confided to his care, has been frequently known to increase the internal pressure of a large boiler many thousand pounds beyond the resistance to which it was originally proved. To prevent a recurrence of those accidents, which first drew the attention of the legislature to this important part of the engine, it appears advisable to inclose the safety-valve in an iron case, of which a section is annexed.