THE METHOD OF MEANS. 423 



side with a pressure varying according to the depth, 

 but always in a direction perpendicular to the side. 

 We may then conceive the whole pressure as exerted 

 on one point, which will be one-third from the bottom 

 of the cistern, and may be called the Centre of Pressure. 

 The Centre of Oscillation of a pendulum, discovered by 

 Huyghens, is that point at which the whole weight of 

 the pendulum may be considered as concentrated, without 

 altering the time of oscillation (see p. 370). Similarly 

 when one body strikes another the Centre of Percussion 

 is that point in the striking body at which all its mass 

 might be concentrated without altering the effect of the 

 stroke. Mathematicians have also described the Centre 

 of Gyration, the Centre of Conversion, the Centre of 

 Friction, &c. 



We ought however carefully to distinguish between 

 those circumstances in which an invariable centre can 

 be assigned, and those in which it cannot. In perfect 

 strictness, there is no such thing as a true invariable 

 centre of gravity. As a general rule a body is capable 

 of possessing an invariable centre only for perfectly 

 parallel forces, and gravity never does act in absolutely 

 parallel lines. Thus, as usual, we find that our concep- 

 tions are only hypothetically correct, and only approxi- 

 mately applicable to real circumstances. There are indeed 

 certain geometrical forms, called Cenfaobaric\ such that 

 bodies of that shape would attract each other exactly 

 as if the mass were concentrated at the centre of gravity, 

 whether the forces act in a parallel manner or not. 

 Newton shewed that uniform spheres of matter have 

 this property, and this truth proved of the greatest im- 

 portance in simplifying his calculations. But it is after 

 all a purely hypothetical truth, because we can nowhere 

 meet with, nor can we construct, a perfectly spherical 



1 Thomson and Tait, 'Treatise on Natural Philosophy/ vol. i. p. 394. 



