6:2 



Mr. Gilchrist on the Balance 



[July 



are simultaneous movements. But the enigma is solved the moment we 

 comprehend that there are really two balances concerned, which have 

 a common counterpoise— namely, the mercury. The one balance is 

 composed of this and the atmosphere, — two fluids, the easy mobility 

 of the particles of which allows a ready mutual counterpoise ; the other 

 balance is the visible one, of which the mercury and its containing tube 

 at one end, and weight at the other, are counterpoises. Comprehend- 

 ing this, it is easy to trace what obtains, when an increase of atmos- 

 pheric pressure occurs ; the air forces into the tube mercury equal in 

 quantity to its increased weight, and thus effects a counterpoise. But 

 this increase of mercury destroys the equilibrium previously existing 

 between it and the weight ; the former preponderating, its end of the 

 balance necessarily descends, but while the tube is descending, the 

 mercury and air preserve their equilibrium, so that the former remains^ 

 in the case supposed, at 29.5 inches above the surface of that in the cis- 

 tern, while the tube slides over it in the manner already described. It 

 may be said that this explanation offers an objection to the balance 

 barometer more serious than the one advanced by Mr. Taylor ; inas- 

 much as his shows only an intermitting pull, while this shows a con- 

 stant one. If no check to the unlimited descent of the tube existed, 

 this would hold good ; but let us now attend to what occurs to the 

 lower portion of the tube. Here, as Mr. Taylor remarks, " it must be 

 " recollected that the part of the tube immersed in the quicksilver, les- 

 " sens the weight, imposed upon this end of the balance, to the amount 

 li of the weight of an equal volume of quicksilver." When, therefore, 

 atmospherical pressure causes a given volume of mercury to enter the 

 tube, this descends from increased weight, and necessarily dips into 

 the mercury in the cistern : but this dipping causes the tube to lose so 

 much of its weight as is equal to the volume of mercury Undisplaces ;— • 

 so when the volume of mercury thus displaced equals that which en- 

 tered the tube, an equilibrium is effected ; the tube has lost the weight 

 it gained ; a further dipping would make it lighter than the counter- 

 weights, because, the mercury remaining at 29.5, no additional weight 

 is added by the column; while the further descent of the tube into the 

 mercury makes it as much lighter as the volume of quicksilver it dis- 

 places ; a further descent of the tube is thus prevented, and if it forced 

 down further than the point at which this equilibrium occurs, it would 

 rise to that point when the foreign force thus depressing it is removed. 

 Here, then, in this principle of specific gravity, we observe a third ba- 

 lance concerned in the balance self-registering barometer ; which, like 

 the one consisting of the atmosphere and the mercury, is of the nicest 

 sensibility ; and the third balance discharges the important function 

 of preventing the unlimited descent of the tube. The above are the 

 reasons that induce me to think that the principle of my ♦plan is correct. 



