82 Automatic Weighing at the Royal Mint. 



each machine at a moment's notice. The withdrawal of the 

 pressure permitted them to resume their duties at the same 

 brief notice. 



In a remote comer of the room was placed the direct, though 

 in itself secondary, motive power — a small atmospheric engine. 

 This was constructed so as to resemble very closely a high- 

 pressure steam engine. It had cylinder, piston, slides, fly- 

 wheel, and governor. Beneath the cylinder, and forming its 

 bed-plate indeed, was a vacuum-chamber of considerable di- 

 mensions. This could be exhausted by an air-pump, with 

 which a two-inch pipe connected it ; whilst the extent of rare- 

 faction within it was made controllable by means of a steelyard 

 relief- valve, and a barometer-gauge placed on the exhaust-pipe. 

 The air-pump was the same which gave motion to the pneu- 

 matic apparatus of the coining-presses. It was on the double- 

 acting principle — that is, it exhausted in both up and down 

 strokes, and had many peculiarities to distinguish it from 

 ordinary air-pumps. The writer may fairly take some credit 

 for its invention and introduction to the Eoyal Mint. Keturn- 

 ing to the atmospheric motor of the automatom weighing 

 balances, it must be further said that when its vacuum-chamber 

 was exhausted, and the opening of a cock in the exhaust-pipe 

 caused it instantaneously to be so, it was only necessary 

 further to move the fly-wheel slightly (so as to turn the crank 

 of the engine past the centre) in order to put it in motion. 

 A stream of air from the room rushed immediately through a 

 small brass tube, having a trumpet-mouth, into the cylinder, 

 and acted upon the piston, as steam from the boiler in an or- 

 dinary engine would act. Fastened to the arms of the fly- 

 wheel was a pulley, and a strap from this passing round a 

 similar pulley gave motion to the overhead shaft. 



We have gone thus minutely into a description of the propel- 

 ling arrangements of the weighing balances, because they are 

 unique, and they combine perfect isolation with perfect regu- 

 larity of motion. The varying speed of the general machinery of 

 the establishment cannot affect that of the atmospheric engine 

 and the shafting it drives, because a uniform vacuum is preserved 

 in the vacuum-chamber of the former, and the air exerts a con- 

 stant pressure on its piston. This uniformity is a sine quanon 

 for correct weighing. As the mind of a judge in a court of 

 justice must, if his decisions arc to bo just, bo unswayed by 

 ion or prejudice, so must tho mute judges of the Mint 

 planohets 01 geld or silver be undisturbed in their action, if 

 their sentences are to bo truthful and worthy confirmation. Wo 

 have said that the law of gravitation is infallible; it is so, but 

 it, must be allowed fair play and perfect freedom to ensure in- 

 fallibility. In tho Mint balances it is the ruling power, but 



