1851.] On the Comparative action of Aneroid Bar ometei's. 225 



be an infinitesimal Inertia in the atoms of the gas of the Simpiesometer 

 but this must be to a very small amount, and exists also in the Aneroid. 



We can thus readily conceive why the Simpiesometer should be 

 theoretically the most sensitive instrument. It must have less Inertia 

 and friction to overcome, than the most delicately made Aneroid, in 

 which, however well constructed, there must be, according to the draw- 

 ing, 6 pivots, 1 bow- piece, 2 springs, 2 fulcra, 2 rods, 1 chain, 1 roller, 

 1 collar, 2 levers, and the condensing box-cover to move, or rather 

 (for the sailor's term is the most expressive here) to "give a start to"* 

 before motion takes place ; and all these have their Inertia, friction 

 and some also an attraction of cohesion, however small it be, ready to 

 resist a minute atmospheric change, especially after any repose. 



The fact that the instrument acts as well or better than the Baro- 

 meter during gradual changes, or when carried to the top of a house 

 or hill, seems to me also strongly to confirm this theory (for I beg it 

 may be taken as yet for nothing else), that it is the difference of Inertia 

 which will always render the Simpiesometer the most trust-worthy 

 instrument for a timely indication of a change. For we must recollect 

 that the Inertia of every machine apparently increases with the time 

 of perfect rest, because, it is supposed, a small amount of cohesion 

 takes place. Now when the Aneroid or Barometer are moved from 

 their places, whether to be carried up stairs only, or to the top of a 

 mountain, they are, however carefully handled, jarred sufficiently for 

 the mere vibration of the parts of the Aneroid and the motion of the 

 mercury to overcome that portion of their Inertia which depends on 

 cohesion ; and even if we suppose that the instruments could be moved 

 without any vibration or motion of the mercury, which is impossible, 

 there is still the change of temperature, which is quite sufficient to 

 destroy the minute cohesion of which we are speaking, and diminish 



* If a billiard ball be placed upon a table it may be moved (started) by the 

 smallest feather. If an 18 pound shot replace the billiard ball, a strong quill will 

 bend before the shot is started. This is, for Sailors, a familiar illustration of Inertia. 

 If the ball or the shot be left for some hours it will if measured by instruments be 

 found that a slight additional force is required to move it, because some cohesion has 

 taken place. If a bolt or pin be passed through the balls, and they are suspended 

 like the sheave of a block, they will then oppose both their Inertia and the friction 

 of their pivots to the power which puts them in motion. 



