OF SOLIDS AND LIQUIDS. 247 



if the piston were immersed in the wide ocean 

 itself.* 



* It is owing to the relative weight which particular bodies 

 possess, with relation to the medium by which they are sur- 

 rounded, that advantage is taken of this property, and that 

 it is made instrumental to the most useful purposes. Jt is 

 owing to the tendency downwards, which lead, or iron, 

 possesses, with relation to atmospheric air, that it becomes a 

 power mechanical ; a power which is able to set in motion a 

 number of wheels, so equally and admirably adapted to the 

 nature of the power which causes them to move, as to produce 

 a regular and uniform motion on a dial, or plate, so divided 

 into parts, that a precise knowledge of time may be ascer- 

 tained by the different spaces over which the index is made to 

 move. The effects thus produced the motion which the 

 index describes however regulated it may be by the mecha- 

 nical construction of the machine, is altogether to be referred 

 to the gravity, or weight of the lead, as the prime mover of the 

 whole ; the weight altogether depending on the density of the 

 lead, with respect to the rarity of the atmospheric medium in 

 which it is placed. In order, therefore, that this power should 

 be always the same, an absolute necessity exists, that a same- 

 ness between both should subsist. If the atmospheric me- 

 dium, instead of being the same, should occasionally vary, 

 the relative gravity of the weights will be, consequently, al- 

 tered, if the medium be rarer, the weight will be heavier, 

 and the clock will, consequently, be made to move faster ; 

 and if the atmosphere be more dense, the relative gravity of 

 the lead will be less, and the pull, or drag, on the different 

 wheels will, of course, be diminished ; and by being dimi- 

 nished, the motion of the index will require a longer period 

 in performing its revolution ; it would move slower, if the 

 weight were immersed in water, tban in air ; and in atmos- 

 pheric air, than if suspended in an exhausted receiver. 



