736 Transactions of the Ameiucan Institute. 



perform its delicate and onerous functions with unerring exactitude. 

 At present there are seventeen of these silent but infallible pledges of 

 the accuracy of weight of standard gold and silver coins in the weigh- 

 ing room of the mint. The automatic balances appear almost to 

 be endowed with the faculty of thought. They seem to deliberate, 

 as it were, upon the character of each piece of money submitted to- 

 their arbitration, and to ac<piit or condemn according to the evidence 

 adduced. Each machine is placed upon a low bench or table of cast 

 iron, and at first glance they might be mistaken for so many skeleton 

 clocks covered by glass cases to protect their " works" from dust and 

 humidity. In order to communicate motion to them, a line of small 

 and brightly turned wrought irt>n shafting, supported by neat pen- 

 dants from the ceiling above, spans the whole length of the apartment. 

 The shafting is placed immediately over the machines, and fine gut 

 bands passing around pulleys descend to corresponding pulleys on 

 the driving spindles. The lower series of pulleys are immediately 

 outside the machine ca>e6, through holes in which the spindles 

 turn. A small brass weight lever, attached to each machine, serves 

 to tighten the gut band, so as to give motion to the coin-feeding 

 slides, &c., within the cases. Tiny friction crutches, adjustable by 

 the pressure of the thumb and finger, allow of the engagement or 

 disengagement of each machine at a moment's notice. In a remote 

 corner of the weighing room is placed the motive power. This con- 

 sists of an atmospheric engine, which closely resembles, externally, a 

 high, pressure steam engine. It has its cylinder, piston, slide-valve, 

 jrovernor and fiv-wheel. Beneath the cvlinder, and forming the bed 

 plate upon which it rests, is a vacuum chamber of considerable area. 

 This is exhausted by an air pump, and the extent of rarefaction within 

 the chamber is controlled and regulated by a relief valve and baro- 

 meter gauge. When this atmospheric motor of the automatic 

 machines is required to be placed in action, a stopcock is turned, and 

 a communication is thus made witli the air pump. A stream of air 

 from the room then rushes through a bell-mouthed tube of brass and 

 presses upon the piston of the engine. The rotary motion follows as 

 if steam were the agent employed to effect it. The advantage of 

 this arrangement is that a uniform rate of speed is obtained for the 

 overhead shafting, by the intervention of a strap and pulley, and so 

 for the automatic machines. AVithout that uniformity accurate 

 weighing would be an impossibility. It has been femarked that 

 "those who think twice before speaking once, speak twice the better 



