NOTES. 



503 



NOTE 211, p. 217. The coloured rings are produced by the interference of 

 two polarised rays in different states of undulation, on the principle explained 

 for common light. 



NOTE 212, p. 250. If heat from a non-luminous source be polarised by reflec- 

 tion or refraction at r, fig. 64, the polarised ray r s will be stopped or transmitted 

 by a plate of mica, M I, under the same circumstances that it would stop or 

 transmit the light; and if heat were visible, images analogous to those of 

 figs. 65, 67, &c., would be seen at the point . 



NOTE 213, p. 253. The Rev. John Buchanan, of Charleston, South Carolina, 

 lias recently shown, by ingenious experiments, that the vulture is directed to 

 his prey by the sense of sight alone. 



NOTE 214, p. 305. The class Cryptogamia contains the ferns, mosses, fun- 

 guses, and sea-weeds: in all of which the parts of the flowers are either little 

 known or too minute to be evident. 



NOTE 215, p. 307. Zoophites are the animals which form madrepores, corals, 

 sponges, &c. 



NOTE 216, p. 308. The Saurian tribes are creatures of the lizard or crocodile 

 kind. Some of those found in a fossil state are of enormous size. 



Fig. 72. 



NOTE 217, p. 356. When a stream of 

 positive electricity descends from P to n, 

 fig. 72, in a vertical wire at right angles. to 

 the plane of the horizontal circle AB, the 

 negative electricity ascends from n to P, 

 and the force exerted by the current makes 

 the north pole of a magnet revolve about 

 the wire in the direction of the arrow-heads 

 in the circumference, and it makes the 

 south pole revolve in the opposite direc- 

 tion. When the current of positive elec- 

 tricity flows upwards from M to P, these 

 e ffects are reversed. 



Fig- 73. 



NOTE 218, p. 358. Fig. 73 

 represents a helix or coil of 

 copper wire, terminated by 

 two cups containing a little 

 quicksilver. When the posi- 

 tive wire of a Voltaic battery 

 is immersed in the cup p, and 

 the negative wire in the cup 

 n, the circuit is completed. 

 The quicksilver ensures the 

 connection between the bat- 

 tery and the helix, by conveying the electricity from the one to the other. 



