THE EARTH.

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Moreover, it may be proved that the difference in theattraction thus caused will really he larger than might at firsthe supposed, owing to the increase of density which we haveevery reason to feel sure must exist in the central, as com-pared with the superficial, parts of the Earth . The actualresult is found to be that a body weighs somewhat more thantriyrrth part less at the equator than at the poles, in consequenceof the spheroidal form of the Earth .

In intermediate positions each of these effects will, of course,have an intermediate value. Taken together, they are suffi-cient to cause 1941bs. at the poles to weigh only 1931bs. at theequator ; or 333 lbs. in London to weigh only 332 lbs. at Quito .This statement may be very satisfactorily tested by the use ofan accurate spring balance; or it may be proved by observetions of the oscillations of a pendulum, since these dependupon the effective weight of its bob, and if the bob becomelighter the pendulum will oscillate more slowly. It is, infact, found that a difference of about 2J minutes per diem isthus produced in the time of a clock governed by a pendulum,if it be removed from London to the equator.

Here, as previously in the probable effect of small changesof the eccentricity of the Earth ’s orbit upon the production ofglacial epochs, we have another instance of the remote andindirect connection which astronomy proves to exist betweencauses and effects otherwise apparently dissociated. It is surelya useful, and we would hope a pleasing, intellectual exercise, tofollow the steps by which it is demonstrated, that a clock willgain or lose, or the weight of a given volume of any givensubstance will be increased or diminished, according to thelatitude of the locality in which it may be placed ; and to suchan extent as may suffice, if it be carefully observed, to demon-strate the rotation of the Earth upon its axis.

The rotation of the Earth may also be detected by artilleryexperiments. If a cannon be fired, due north or south, at a verylong range (which with modern ordnance may amount to fiveor six miles), a considerable deviation of its aim will arise, fromthe difference of the velocity with which the Earth ’s rotationcarries the gun towards the east (the whole of which velocity