B2 Mr. A. Day on the Rotation of the Pendulum. 



law. It is therefore the earth that moves, not at the rate of the 

 pendulum nor round the pendulum -wire when at rest as an axis, 

 though this it might do if this were consistent with the observed 

 motions of the stars ; but round another axis pointing to another 

 star, and at a rate coinciding with the apparent rate of the side- 

 real revolutions. It is therefore a sensible proof of another kind 

 than any previously known to exist, and of directly sensible 

 proofs measured by the eye in short intervals of time, the most 

 conclusive in its kind. The value of the whole argument may 

 be thus stated. Either the heavens or the earth rotate once in 

 twenty-four hours, or both move by a joint motion away from 

 each other equivalent to this in amount ; and either of these 

 conditions is abstractly possible, so that no conclusion can be 

 drawn. In this latitude the pendulum rotates in about thirty 

 hours, or the earth in twenty-four; and as there can be no 

 assignable reason why the former should do this without a cause, 

 the rotation of the earth in twenty-four is a fact for the rational 

 being, since it is clear that no supposition of the observed motion 

 being partly in the earth and partly in the heavens will fulfill the 

 second set of conditions. The experiment of the pendulum is 

 therefore very correctly designated as a making visible the earth's 

 rotation, though hardly in the sense popularly attached to it ; 

 and its importance cannot, we think, be overrated. So much 

 then for the second objection ; and I shall now proceed to show 

 that the rotation is to all intents and purposes real, and not 

 merely apparent on the part of the vibrating plane. I have else- 

 where, in a published diagram, shown that the pendulum's 

 actual rotation is the resultant of two sets of continually-exerted 

 forces, one tending to keep the pendulum swaying parallel 

 to itself, the other to make its point of suspension and the line 

 which the pendulum would occupy if at rest rotate obliquely to 

 the axis of the two circles described by the point of suspension 

 and the ball at the mean of the oscillation. Shortly after the 

 publication in question, I constructed a simple mode of illustrating 

 the fact. Cut out two circular discs of card, and make an equal 

 number of cogs or teeth in the rim of each. Paste one of these 

 on a larger circular disc and concentric with it, and cut both 

 through along any radius from the circumference to the centre. 

 If now we draw a diameter on the other toothed disc, and cause 

 it to travel round by placing the teeth of the two discs in one 

 another in the manner of ordinary wheels when acting on each 

 other, but so that the larger disc shall remain motionless, we 

 shall find that this line will have rotated once for one entire 

 revolution of the moving wheel round the circumference of the 

 fixed one. If now we coil up the large disc with its attached 

 rack into the shape of a cone,, so as to hide some of the teeth, 



