190 AJSTNUAL OF SCIENTIFIC DISCOVERY. 



placed in a horizontal position, and made to revolve about its axis. In this 

 case the needle of the compass will always point in the same direction 

 namely, towards the north, and the index card that is fixed to it will be 

 carried round by the motion of the wheel without any rotation about its own 

 axis. But this is a very different motion from that of the moon; and, in fact, 

 if the moon moved round the earth in a manner similar to that just described, 

 ah 1 the parts of its surface would be in succession visible from the latter." 



2. " Take a common compass, and place it at the extremity of one of the 

 arms of a turnstile. When the turnstile has gone half round, look at the 

 compass, and you will find that the northern end of the needle points to the 

 south of the card. By the time the turnstile has got all the way round, the 

 needle will again, as at first, point to the north of the card. Now, here it is 

 very plain that either the needle has moved on its axis round the card, or the 

 card has performed a revolution on that which is the common axis of itself 

 and the needle ; the eye will inform us that the former is not the case, and 

 therefore that the latter must be." 



3. " Take a cup and bah 1 , and marking the latter at four opposite points 

 with the letters 1ST., S., E., and "W., carry it, suspended by its string, round the 

 flame of a candle. You will find that if N". be kept always to the north, the 

 ball consequently remaining without axial motion, the light will fall in suc- 

 cession on W., S., and E., until it reaches N. again. But if you wish N. to 

 be always illuminated, you must turn it continually towards the flame ; in so 

 doing, you will cause an axial rotation of the bah 1 upon its string at each 

 revolution which it performs round the candle." 



4. "A body is said to have no rotary motion when any line drawn in it 

 continually points in the same direction hi space. If the moon had no rota- 

 tion, a line drawn from her centre to any point on her surface would con- 

 tinually point towards the same place in -the heavens that is, towards the 

 same fixed star. A body, on the other hand, is said to have a rotary motion 

 about an axis, when any line drawn through that axis and at right angles to 

 it gradually turns round, so as to point successively to all points of the 

 heavens lying in a great circle. " 



5. " Take a disc of tin for the moon, hollowed a little on one side to make 

 it balance easily on a strong needle stuck point upwards near the end of 

 a bar of wood revolving horizontally. You can hold the disc with your 

 finger while you turn the bar, so as to keep some mark upon the disc facing 

 the axis on which the bar turns, and let it go just before you stop the motion 

 of the bar. In the converse experiment you have only to turn the bar, 

 leaving the disc alone ; and then it will not revolve (except in its orbit), but 

 will present ah 1 its circumference in succession to the axis of the bar thus 

 showing that an additional force was necessary to make the- moon turn on its 

 axis, besides turning round the earth." 



At the last meeting of the British Association, the subject was brought up 

 by Dr. Whewell, who presented the following paper : 



The moon's motion may be described, in one way among others, by saying 

 that in each month she revolves about the earth nearly in one plane, turning 

 always the same face to the earth. But a body rigidly fastened to a rigid 



