THE POPTJLAB EDUCATOR 



'mantelpieces.;' when exposed to the air, the softer parts, which 

 are inmediately between the stems, weather away, leaving the 

 surface of the rock as if the fossils had been stuck on to 

 it, as in the illustration of encrinital limestone (Fig. 75). The 

 heads of the animals which the stem supported were a corona 

 of tentacules, with a mouth at the centre. These arm-like 

 feelers waved about in the waters, bringing into their mouth 

 the prey which they enclosed. As specimens of these encri- 

 nites, we give the Cyathocrinites planus (Fig. 76) and the 

 Woodocrinus (Fig. 77). 



Fossils of the sea-urchins are also found, but these palseozoic 

 echini had more plates than are found in specimens of their 

 existing representatives, and their detached spines are of common 

 occurrence. 



The Mollusca appear In great force. The trilobites still 

 struggle for existence, and are represented by two members of 

 the race Phillipsia (Fig. 78) and Griffithides. The crustaceans 

 have other representatives, such as the Eurypterus (Fig. 79). 

 The fish of the preceding age, of course, have not become 

 extinct, and many perfect fossils remain ; moreover, species 

 variously formed are continually met with, which indicate 

 the existence of other fish armed with defensive weapons. 

 These sharp fin-spines were formidable, as may be judged from 

 the appearance of the Pleuracanthus (Fig. 80) and the Ctena- 

 canthus (Fig. 81). 



There is also evidence of the presence in the carboniferous 

 seas of fish, the roofs of wh^se mouths were armed with palatal 

 plates. We should have been at a loss to have even conjectured 

 the use of these smooth, black, and oblong hard pieces of enamel, 

 had it not been that in Australia there is a shark the Port 

 Jackson shark whose mouth is paved with these very plates. 

 The creature lives on crustaceans, and to break, for instance, 

 the shell of a crab, sharp and pointed teeth would be useless ; 

 so that he is furnished with plates by which he crushes and 

 breaks the hard coats of his food, and BO lives. These fossils 

 were vulgarly called fossil leeches. Two of these palatal 

 plates of the Placoid sharks we figure, namely, Cochliodus 

 contortus (Fig. 82) and Psammodus porosus (Fig. 83). The chief 

 fossils of the period are also represented in the illustration in 

 the preceding page. 



Some of the above fossils are very numerous : for example, 

 the Productus sometimes all but forms the whole mass of the 

 rock, which is then called a Productus limestone. 



The chambered shells, as represented here by the Euomphalus, 

 the Goniatites, and the Orthoceras, have many peculiarities 

 which command notice. 



The Bellerophon has no chambers, it is like the living 

 Argonaut ; the genus is not found after this epoch. 



The Euomphalus is chambered, but there is no connection 

 between the chambers, so that as the animal grows it builds up 

 its shell behind it, throwing a wall or septum (Latin, septum, a 

 partition) from side to side. 



The Nautilus tribe do the same, only they have a pipe which 

 communicates with each of the chambers : in the nautilus this 

 siphuncle is in the centre, whereas in the ammonite tribe it is 

 marginal, or runs round the outside edge of the shell. The 

 creature is a cephalopod, its arms are out of its head ; as it 

 moves on the bottom of the sea seeking its food, if it dragged 

 its delicate shell after it, it would be bruised and damaged ; 

 hence it passes, by means of its siphuncle, either water or air 

 into the emptj chambers, and so makes its shell of the same 

 specific gravity as the water around it, and so it just swims on 

 the surface of the water. 



The Orthoceras (Fig. 84) is a nautilus unrolled, having a 

 siphuncle passing down the centre of the cells. 



The Goniatites (Fig. 85), on the other hand, is of the Ammo- 

 nite type ; the siphuncle is marginal. 



Some of the mountain limestones are very beautiful. . The 

 Terelratula lastata (Fig. 86) frequently exhibits the original 

 coloured stripes which ornamented the living shell. 



The Aviculopecten (Fig. 87) and the Pleurotomaria (Fig. 88) 

 also exhibit the same phenomenon, though in not so marked a 

 degree. 



The presence of these colours indicates that the sea in which 

 they lived was not more than fifty fathoms deep. 



Other notable fossils of the period are Productus (Fig. 89), 

 Spirifer (Fig. 90), Murchisonia (Fig. 91), Euomphalus (Fig. 92), 

 and Bellerophon (Fig. 93] 



ELECTRICITY. IX. 



ATMOSPHERIC ELECTRICITY FRANKLIN'S KITE EXPERIMENT 



CROSSE'S APPARATUS LIGHTNING CONDUCTORS. 

 WE have now to notice some of the main effects of Atmo- 

 spheric or Aerial. Electricity. These are very interesting and 

 very important, as it seems highly probable that by inquiring 

 into them we shall ultimately understand much more of the 

 phenomena of the weather than we do at present. 



The many points of resemblance between lightning and elec- 

 tricity were noticed by early experimentalists. The flash of the 

 electric spark resembles faintly a flash of lightning. A loud 

 sound accompanies each, and both have the power of setting on 

 fire inflammable bodies. Lightning destroys animal life, rends 

 bad or insufficient conductors, and is attracted by the pointed 

 tops of trees or buildings ; and all these effects may be pro- 

 duced on a small scale by the agency of electricity. We easily 

 see, then, the great resemblance. It was left, however, for the 

 celebrated Franklin to prove their identity, and, by drawing 

 down the lightning from the clouds, to render his name for 

 ever famous in the annals of electrical science. 



About the commencement of the year 1752 he began to 

 investigate the subject, and formed the idea of erecting, on a 

 convenient building, a tall insulated conductor, terminating in 

 a point, by which he expected to be able to collect the elec- 

 tricity from passing clouds. He was unable just at the time 

 to carry out his purpose ; ho had, however, given a full expla- 

 nation of his ideas in a letter to a friend, and acting upon this 

 a French electrician succeeded in obtaining sparks from an 

 apparatus he had prepared in accordance with these plans. 

 Franklin was now tired of waiting for the erection of a building 

 suitable for his purpose, and the idea occurred to him of trying 

 his experiment with a kite. He accordingly made a framework 

 of two thin laths crossing one another, and fixed the four corners 

 of a silk handkerchief to their extremities. A tail was affixed 

 in the usual way, and a loop for the string to be fastened to. 



In order to collect the electricity he fixed a pointed wire to 

 the upper end of one of the laths, and connected it with the 

 string of the kite. The other end of the string he fastened to 

 the ring of a door-key, and fixed to this a piece of silk ribbon, 

 so that he might hold it without the electricity escaping, and 

 could draw sparks from the key. 



Thus equipped he went out one stormy day, accompanied only 

 by his son, and having raised the kite on a common near Phila- 

 delphia, awaited with almost breathless anxiety the result. For 

 some time no effects whatever were seen ; there was not the 

 slightest appearance of any electrical disturbance; after a 

 while, however, he observed some of the fibres of the cord 

 bristling up ; and on applying his knuckle to the key, to his 

 intense delight he drew from it a spark. Soon afterwards a 

 heavy shower of rain fell, and the cord having thus become 

 wetted, its conducting power was much increased, and a rapid 

 succession of sparks was given off from the key, by which a 

 Leyden jar was charged, and other experiments performed 

 which proved beyond doubt the absolute identity of lightning 

 and electricity. 



Franklin was thus the real discoverer of this fact, though 

 some have attempted to claim this honour for the French 

 philosopher we have referred to, who was named Dalibard, on 

 the ground that he erected the first apparatus by which the 

 experiment was performed. 



Since this time the experiment has been repeated over and 

 over again in a great variety of ways. Frequently a fine wire 

 is woven into the string, and greatly increases its conducting 

 power. When this is not done it should be moistened with salt 

 and water. Captive balloons have likewise been employed to 

 draw down the electric fluid. The utmost caution is, however, 

 requisite in conducting these experiments, as fatal accidents 

 have arisen in carrying them out. Not very long after the date 

 of Franklin's experiment, Professor Eichmann had erected an 

 apparatus at St. Petersburg for collecting in this way the 

 atmospheric electricity, the wires being brought into his labora- 

 tory. One day he was explaining it to a friend, just as a storm 

 was coming on, and all at once a loud report was heard, and the 

 electricity issued almost in the form of a ball of fire, killing him 

 on the spot, and striking his friend senseless to the groiind ; 

 many of the things in the room were at the same time shaken 

 or destroyed. 



