298 



NATURE 



[Feb. I, 1877 



greater than the distance of the comers from the centre. 

 The friction of the fly-wheel would, if the bearing-edge 

 were circular, cause the case to roll along on it like a 

 hoop, and it is to prevent this effect that the curved poly- 

 gonal form described above and represented in the draw- 

 ing is given to the bearing-edge. 



To spin the solid gyrostat a |Mece of stout cord about 

 forty feet long and a place where a clear run of about 



F/ff.3 



sixty feet can be obtained are convenient. The gyrostat 

 having been placed with the axis of its fly-wheel vertical, 

 the cord is passed in through an aperture in the case, two- 

 and-a half times round the bobbin-shaped part of the 

 shaft, and out again at an aperture on the opposite 

 side. Having taken care that the slack cord is 

 placed clear of all obstacles and that it is free from 

 kinks, the operator holds the gyrostat steady so that 

 its case is prevented from turning, while an assistant 

 pulls the cord through by runnmg, at a gradually in- 

 creasing pace, away from the instrument, while hold- 

 ing the end of the cord in his hand. Sufficient tension is 

 applied to the entering cord to prevent it from slipping 

 rourd on the shaft. In this way a very great angular 



I'if/.A. 



velocity is communicated to the fly-wheel, sufficient, in- 

 deed, to keep it spinning for upwards of twenty minutes. 

 If when the gyrostat has Iseen spun it be set on its 

 bearing edge with the centre of gravity exactly over the 

 bearing point, on a smooth horizontal plane such 



piece of plate-glass lying on a table, it will continue appa- 

 rently stationary and in stable equilibrium. If while it is 

 in this position a couple round a horizontal axis in the 



plane of the fly-wheel be applied to the case, no deflec- 

 tion of this plane from the vertical is produced, but it 

 rotates slowly round a vertical axis. If a heavy blow with 

 the fist be given to the side of the case, it is met by what 

 seems to the senses the resistance of a very stiff elastic 

 body, and, for a few seconds after the blow, the gyrostat 

 is in a state of violent tremor, which, however, subsides 

 rapidly. As the rotational velocity gradually diminishes, 

 the rapidity of the tremors produced by a blow also 

 diminishes. It is very curious to notice the tottering con- 

 dition, and slow, seemingly palsied, tremulousness of the 

 gyrostat, when the fly-wheel has nearly ceased to spin. 



In the liquid gyrostat the fly-wheel is replaced by an 

 oblate spheroid, made of thin sheet copper, and filled with 

 water. The ellipticity of this shell in the instrument ex- 

 hibited is =[^5 t^^^ ^s ^° ^y> ^^ equatorial diameter 

 exceeds the polar by that fraction of either. It is pivoted 

 on the two ends of its polar axis in bearings fixed in a 

 circular ring of brass surrounding the spheroid. This 

 circle of brass is rigidly connected with the curved poly- 

 gonal-bearing edge which lies in the equatorial plane of 

 the instrument, thus forming a frame-work for the support 

 of the axis of the spheroidal shell. In I'ig. 3 a section is 

 represented through the axis to show the ellipticity, and 

 Fig. 4 gives a view of the gyrostat as seen from a point in 

 the prolongation of the axis. To prevent accident to the 

 shell when the gyrostat falls down at the end of its spin, 

 cage bars are fitted round it in such a way that no plane 

 can touch the shell. 



The method of spinning the liquid gyrostat is similar 

 to that described for the solid gyrostat, differing only in 

 the use of a very much longer cord and of a large 

 wheel for the purpose of pulhng it. The cord is first 

 wound on a bobbin, free to rotate round a fixed pin. The 

 end of it is then passed two-and-a-half times round the 

 little pulley shown in the annexed sectional drawing, and 

 thence to a point in the circumference of the large wheel 

 to which it is fixed. An assistant then turns the wheel 

 with gradually increasing velocity, while the frame of the 

 gyrostat is firmly held, and the requisite tension apphed 

 to the entering cord to prevent it from slipping round the 

 pulley. William Thomson 



REMARKABLE PLANTS 

 I.— The Compass- Plant. 



Look at this delicate plant that 1 fts its head from the meadow, 

 See how its leaves all point to the north, as true as the magnet ; 

 It is the compass-plani that the finger of God has suspended 

 Here on its fragile stalk, to direct the traveller's journey 

 Over the sea-l>ke, pathless, limitless waste of the desert " 



Longfellow's Evangeline. 



r 



has long been known that there grows on the 

 prairie-lands of the south-western part of the United 

 States of America, especially Texas and Oregon, a 

 plant which has the peculiar property of turning its 

 leaves towards the north, and which hence serves as a 

 magnet to the traveller when no other means is available 

 of ascertaining the points of the compass. It is probable, 

 however, from Longfellow's description of it as a "deli- 

 cate plant " on a " fragile stalk," that he never saw it 

 growing. The Compass-plant is a member of the enor- 

 mous natural order Compositae, known to botanists as 

 Siiphium laciniatum. It is described as a stout perennial 

 plant from three to six feet in height, with ovate, deeply- 

 pinnatifid leaves and large yellow heads of unisexual 

 flowers, the ray-florets strap-shaped and female, the disc- 

 florets tubular and male. It is also known as the pilot- 

 weed, polar-plant, rosin-weed, and turpentine-weed, the 

 two last names being derived from the abundant resin 

 exuded by the stem ; and is occasionally to be seen in 

 English gardens. 



The " polarity " of the leaves of this singular plant has 

 long been familiar to hunters and other denizens of the 

 prairie, who, " when lost on the prairies in dark nights, 



