41 



below the water line; the stem of the thermometer being curved at 

 its lower extremity to correspond with the tube. The scale was gra- 

 duated as an indicator of pressure in accordance with the results of 

 the experiments made by the Franklin Institute of Pennsylvania. A 

 comparison of its indications with those of a very well made and 

 carefully graduated safety valve, attached to the same boiler, had 

 proved entirely satisfactory. 



Dr. John Locke, of Cincinnati, a member of the National In- 

 stitution, visiting the Society, was invited to make a communi- 

 cation explanatory of the " Safety Guard" of Mr. Cadwal- 

 lader Evans, of Pittsburg, of which he presented a working 

 model to the notice of the Society. 



After some observations on the causes of accidents to high pressure 

 boilers with flues, most of which were referred either to over pres- 

 sure of saturated steam, or to over heating the flues when the water 

 is low ; and a brief historical sketch of the introduction of fusible al- 

 loys in aid of the safety valves and gauge cocks; Dr. Locke called 

 the attention of the Society to the model of Mr. Evans's invention. 



In this, the fusible alloy is placed in the bottom of an iron tube, 

 which is inserted into the boiler and attached to it steam-tight by a 

 flange at the top, or outer extremity, while the end containing the fu- 

 sible metal is placed in contact with the upper part of the flue, so as 

 to receive the greatest heat of the part first exposed by low water. A 

 key, like the key of a common lock, pivoted in the alloy, continues 

 fixed while the alloy remains solid, but is free to turn as soon as it 

 fuses. The stem of the key, passing through a collar, terminates on 

 the outside in a cylindrical head or pulley. To this a chain is fixed, 

 which, after being wound round the cylindrical head, passes over a 

 simple pulley at the end of the lever of the safety valve, and there sus- 

 pends the weight. 



While the alloy remains solid, the action of this weight is the same 

 as if it were attached to the lever itself, as in the common arrange- 

 ment of the safety valve. When the temperature of the " guard" 

 reaches the point for which the alloy was composed; in consequence 

 either of the excessive temperature of the steam that surrounds it, or 

 of the water sinking below the top of the flue on which it rests; the 

 alloy melts: the weight acting on the chain turns the loosened key, 

 the chain is unwound, and the weight, descending upon a platform 

 placed to receive it, relieves the safety valve of its load. 



The " safety guard" has the recommendation of great simplicity, 



