^Q BULLETIN 173, U. S. NATIONAL MUSEUM 



Flat panel model of a beam steam engine, made of lead fitted in a wooden 

 case. Not identified. U.S.N.M. no. 308730. 



MILLER MERCURY MOTOR, 1877 



U.S.N.M. no. 308696; original patent model; transferred from the United States 

 Patent OflBce; not illustrated. 



This model was submitted with the application for the patent 

 issued to Thomas Davidson Miller, of Pittsburgh, Pa., November 6, 

 1877, no. 196759. 



The model represents a boiler, a condenser, and an overshot wheel, 

 connected with suitable piping so that mercury placed in the boiler 

 will be sublimated there and the fumes will rise to the condenser 

 where they will be condensed. From the condenser the liquid mer- 

 cury runs over the buckets of the wheel where the weight of tho 

 mercury is employed in turning the wheel. Suitable sheathing about 

 the wheel collects the mercury and returns it to the boiler, which it 

 enters by reason of its weight. 



STEAM-ENGINE VALVES AND VALVE GEARS 



In 1769 James Watt discovered that a saving in steam could be 

 effected in a steam engine by cutting off the supply of steam early 

 in the stroke and permitting the steam to complete the stroke ex« 

 panding. This principle was first used practically in 1776 and was 

 patented in 1782. After about 1800 many valves and valve gears 

 were developed to jDermit the steam to be cut off at any point in 

 the stroke. These took the various forms of separate steam valves 

 that could be closed at any time relative to the position of the piston 

 and the exhaust valves; valve gears to vary the cut-off by varying 

 the valve actuating mechanism relative to the position of the engine 

 crank; and independent cut-off valves that operated to cut oft' the 

 supply of steam to nonvariable valves of simple forms. All these in 

 their original form were set by hand for the most economical cut-off 

 for the speed and load at which the engine was to operate. In 1834 

 Zachariah Allen constructed one of the earliest forms of valve gears 

 in which an engine governor was used to determine the point at which 

 an independent cut-off valve would cut off the supply of steam. The 

 next step was the invention of the drop cut-off, or detachable valve 

 gear, in which a poppet steam valve was raised by a catch that 

 €ould be thrown out at the proper moment by a wedge or some other 

 detaching device with which it came in contact as it rose with the 

 opening valve. The wedge was adjustable so that the valve could 

 be detached and let fall to its seat at any point in the stroke. The 

 invention of this device is generally credited to Frederick E. Sickels, 

 ^'ho patented it in 1841 (see below), though Peter Hogg, of New 

 York, N. Y., also claimed the invention. The drop cut-off provided 



