AMERICAN INSTITUTE. 



703 



usual, and they work more economically when fully loaded than 

 when working under power. Another objection raised to high 

 pressure engines is, that in consequence of the great pressure, 

 there is more leakage of steam at the piston and joints of the en- 

 gine, than with low pressure. In answer to this it may be stated 

 that the joints are of much smaller dimensions in the high than 

 the low pressure engine. The third objection raised to high 

 pressure engines is that they do not realize the advantages of the 

 vacuum obtained in condensing engines. This objection loses in 

 weight, because the surface of the piston becomes proportionately 

 less as the elasticity is increased, and therefore the loss of the 

 vacuum is less to be felt. 



The fourth objection is, that a greater consumption of oil and 

 grease is required for lubrication of the piston, piston rod, and 

 valve apparatus, than, tor engines of low pressure. This objec- 

 tion is sometimes enhanced by the assertion, that the grease be- 

 comes volatilized at the temperature of steam of very high 

 pressure. ^ 



Now it is well known that animal as well as vegetable fat only 

 boils at a temperature of 220° Reaumur (527° Fahrenheit,) and 

 does not, like water, volatilize at a heat under that of ebulition. 

 It can scarcely be perceived how any loss can take place by 

 volatilization in engines working up to ten atmospheres; where 

 only half the above-named heat exists; and this pressure should 

 never be exceeded, even working up to forty atmospheres; not the 

 eightli part of what is necessary for low pressure engines is re- 

 quired A high pressure engine requires much less lubrication 

 for the packings of the piston and piston rod than a low pressure 

 one. 



All boilers should be constructed in such a manner that if they 

 explode they may not be dangerous ; this condition has been ap- 

 proximated by the invention and application of tubular boilers; 

 but they are seldom used for low pressure steam, being particu- 

 larly adapted for high pressure ; the higher the pressure, the 

 smaller are the bubbles of vapor, and the easier their transmis- 

 sion through the water. When, after a fire is lighted under a 

 high pressure boiler, the water first begins to boil, the steam is 



