CONTRIBUTIONS TO SCIENCE. 501 



four quadrants, coloured with vermilion, chrome yellow, 

 emerald green, and ultramarine. These are bright colours, 

 and if the vermilion is good they combine into a grayish tint 

 when the revolution is about the axle, and burst into bril- 

 liant colours when the axis is disturbed. It is useful to 

 have some concentric circles, drawn with ink, over the colours, 

 and about twelve radii drawn in strong pencil lines. It is 

 easy to distinguish the ink from the pencil lines, as they 

 cross the invariable axis, by their want of lustre. In this 

 way the path of the invariable axis may be identified with 

 great accuracy, and compared with theory." If the top 

 revolve about its axle, the whole disc will appear gray, but 

 when the axle moves, the quadrants in which the " invariable 

 axis" lies will be indicated by a circle of the corresponding 

 colour appearing in full purity. Whenever the invariable 

 line crosses an ink or pencil line, a dark ink dot or lustrous 

 pencil dot will appear at the centre of the circle, and when 

 it passes into another quadrant the circle will contract, then 

 change its colour, and expand again. The use of the nine 

 screws is thus indicated by Professor Maxwell : " There 

 must be three adjustments to regulate the position of the 

 centre of gravity, three for the magnitude of the moments of 

 inertia, and three for the directions of the principal axes 

 nine independent adjustments, which may be distributed as 

 we please among the screws of the instrument." A full 

 account of the top and the mode of using it, together with a 

 note on the rotation of the earth, will be found in the Trans- 

 actions of the Eoyal Society of Edinburgh, vol. xxi., from 

 which the above quotations have been taken. 



5. In 1610 Galileo turning his telescope to Saturn, ob- 

 served a projection on each side of the planet, which led him to 

 conclude that " Saturn consists of three stars in contact with 

 one another." In 1659 Huyghens discovered the true nature 

 of these appendages, which, by their continually varying 

 forms, had puzzled previous astronomers, and concluded that 

 Saturn is girded with a thin flat ring inclined to the ecliptic ; 

 its external diameter being about 2J times that of the 



