ON A XEW FOEM OF THE PLATOMETER. 233 



motion of which the instrument is capable. The forms of the rolling parts which 

 I considered were 



1. Two equal spheres. 



2. Two spheres, the diameters being as 1 to 2. 



3. A cone and cylinder, axes at right angles. 



Of these, the first combination only suited my purpose. I devised several modes 

 of mounting the spheres so as to make the principle available. That which I 

 adopted is borrowed, as to many details, from the instruments already con- 

 structed, so that the originality of the device may be reduced to this principle 

 The abolition of slipping by the use of two equal spheres. 



9. The instrument (Fig. 1) is mounted on a frame, which rolls on the two 

 connected wheels, MM, and is thus constrained to travel up and down the 

 paper, moving parallel to itself. 



CH is a horizontal axis, passing through two supports attached to the 

 frame, and carrying the wheel K and the hemisphere LAP. The wheel K rolls 

 on the plane on which the instrument travels, and communicates its motion to 

 the hemisphere, which therefore revolves about the axis AH with a velocity 

 proportional to that with which the instrument moves backwards or forwards. 



FCO is a framework (better seen in the other figures) capable of revolving 

 about a vertical axis, Cc, being joined at C and c to the frame of the instru- 

 ment. The parts CF and CO are at right angles to each other and horizontal. 

 The part CO carries with it a ring, SOS, which turns about a vertical axis Oo. 

 This ring supports the index -sphere Bb by the extremities of its axis Ss, just 

 as the meridian circle carries a terrestrial globe. By this arrangement, it will 

 be seen that the axis of the sphere is kept always horizontal, while its centre 

 moves so as to be always at a constant distance from that of the hemisphere. 

 This distance must be adjusted so that the spheres may always remain in con- 

 tact, and the pressure at the point of contact may be regulated by means of 

 springs or compresses at O and o acting in the direction OC, oc. In this way 

 the rotation of the hemisphere is made to drive the index-sphere. 



10. Now, let us consider the working of the instrument. Suppose the arm 

 CE placed so as to coincide with CD, then O, the centre of the index-sphere 

 will be in the prolongation of the axis HA. Suppose also that, when in this 

 position, the equator bB of the index-sphere is in contact with the pole A of 

 the hemisphere. Now, let the arch be turned into the position CE as in the 



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