April 13, 1876] 



NATURE 



479 



reactions, whenever heat is communicafed from a surface to a 

 gas, and vice versd, and the connection between these forces and 

 the motion caused by heat and light falling on bodies in vacuo. 



Having obtained one of the beautiful little "Light-Mills" 

 constructed by Dr. Geissler, of Bonn, the author was in a 

 position to make quantitative measurements of the effecta pro- 

 duced, and of the force producing them. 



In the first place, with r^aid to the sufficiency of the residual 

 air to cause the motion. It was found that this air is, with the 

 exception of the friction of the pivot, which is found to be so 

 small as to be inappreciable, the sole cause of the resistance 

 which the mill experiences, of the limit which is imposed on its 

 speed for such intensity of light, and of the rapidity with which 

 it comes to rest when the light is removed. The law of resist- 

 ance, as determined by careful measurements, is foimd to agree 

 perfectly with the resistance which highly rarefied air would 

 offer to its motion ; and this law is distinctly special in its cha- 

 racter, being proportional to the velocity at low speeds, and 

 gradually tending towards the square of the velocity as the speed 

 increases. 



Having established the fact that there is sufficient air in the 

 mill (and Mr. Crookes's behaves in the same manner as this 

 mill) to balance, by its resistance, the force which moves the 

 mill, it is argued that all question as to the sufficiency of the air 

 to cause the forces is removed. What the air can prevent it can 

 cause. 



As regards the possibility of the motion being in any way the 

 direct result of radiation. This supposition the author had pre- 

 viously shown to be directly contradicted by the fundamental 

 law of motion that action and reaction are equal. A cold body 

 runs away from a hot body, while, if free to move, the hot body 

 will run after the cold body, showing that the force does not act 

 from body to body, but that each body propels itself through the 

 surrounding medium in a direction opposite to its hottest side, 

 the effisct of one body on the other being due solely to the dis- 

 turbance which it causes in the equilibrium of temperature. 



Besides proving that the force acts between the vanes of the 

 mill and the medium immediately surrounding them Dr. Schuster's 

 experiments furnish a quantitative measure of the actual force. 

 From this measure it is sho^vn on theoretical grounds that the 

 difference of temperature on the two sides of the vanes neces- 

 sary to cause heat-reactions of this magnitude could not be less 

 than 1^7, while the probability is that it is considerably more. 



In order to apply this test and see how far the actual difference 

 of temperature in Dr. Schuster's experiments correspond with 

 that deduced from the theory, a new photometer was devised by 

 the author with an immediate view of measuring the difference of 

 temperature caused by light on a black and a white surface. 



Of two thin glass globes, 2.\ inches in diameter, connected by a 

 syphon tube \ inch internal diameter, one was blackened with 

 lamp-black on the inside over one hemisphere and the other was 

 whitened with chalk in a similar manner, the two clean faces of 

 the globes being turned in the same direction. Oil was put in the 

 tube and the globes were otherwise sealed up. Any light which 

 enters through the clean faces is received on the black and white 

 surfaces, and the air in the globes expands in accordance with 

 the difference of temperature which they attain, moving theoU in 

 the tube. A motion of ^ an inch on the part of the oil shows a 

 difference of 2^ '2, in the temperature of the air within the globes. 



The instrument so constructed is exceedingly delicate, and will 

 show a difference in the intensity of light sufficient to make one 

 revolution per minute difference in the speed of the milL 



Measured with this instrument, the difference of temperature 

 caused by the light necessary to give the mill 240 revolutions per 

 minute does not exceed 24*', and is probably less than this, which 

 shows that the theoretical difference of heat necessary to cause 

 the heat-reactions is well within the difference as actually mea- 

 sured, leaving an ample margin for error in the methods of 

 approximation used in the calculation. 



In concluding the paper the author claims to have set at rest 

 the only point respecting the explanation of the motion caused 

 by heat, which remained doubtful after he had discovered that, 

 according to the kinetic theory, the communication of heat to a 

 gas must cause a force reactionary on the surface, viz., whether 

 this reaction was adequate in amount to cause the results seen to 

 take place. 



He adds a stiggestion as to a new form of light-mill to have 

 vanes inclined like the sails of a windnyll, and not havirjg one 

 side white and the other black, like the light-mills at present 

 constructed. Arguing that the forces act perpendicularly to the 

 surface, and in a direction independent of that from which the 



light comes ; so that such a mill would turn like a windmill with 

 the full and not merely the differential effect of the light. Such a 

 mill, he concludes, would furnish another test as to whether or 

 not the force is directly referable to radiation. 



Geological Society, March 22. — Prof. P. Martin Duncan, 

 F.R.S., president, in the chair. — Frank Campion, Henry J. 

 Gardiner, Henry Percy Holt, Lord Rosehill, Harold UnderhiU, 

 Frederick Thomas Whitehead, and Thomas Wrightson were 

 elected Fellows of the Society. — On the Triassic strata which 

 are exposed in the cliff sections near Sidmouth, and a note on 

 the occurrence of an Ossiferous zone containing bones of a Laby- 

 rinthodcm, by H. J. Johnston Lavi«. The author described the 

 base of the cUffs east of Sidmouth as composed of the marl 

 which is the uppermost subdivision of the Trias in South Devon, 

 capped in Littlecomb Hill and Dunscomb Hill by greensand amd 

 chalk, and in Salcombe Hill by greensand alone. In the valley 

 of the Sid it is largely exposed at the surface. Close to the 

 mouth of the Sid the upper sandstone crops out beneath the 

 marl, forming a cliff overhanging the river. To the west of 

 Sidmouth there is a low projecting cliff, the Chit rock, formed 

 also of the upper sandstone, and at the western end of this is a 

 fault which has given the Chit rock an upthrow of at least 40 and 

 perhaps of 80 feet, since it has no marl capping it, and in its 

 iithological character it resembles the middle part of the Upper 

 Sandstone. To this point the dip is to the east ; but westward 

 of the fault the dip is at first to the west for about half a mile, 

 when the sandstone reappears with an easterly dip, having 

 formed a synclinal curve. It is overlain by marl and greensand 

 in Peake and High Peake Hilk, which are capped with chalk 

 gravels. West of High Peake Hill the sandstone forms the 

 whole cliff. The author described the general characters pre- 

 sented by the Triassic beds in the section under notice, and men- 

 tioned the occurrence at about 10 feet from the top of the sand- 

 stone of a peculiar series of beds, composed of coarse sandstone, 

 containing scattered nodules of marl from the size of a pea to 

 that of a hen's egg, together with numerous fragments of bone, 

 some of which, belonging to a species of Labyrinthodon, would 

 be described by Prof. Seeley. The author mentioned that he 

 had received from the Rev. S. H. Cooke some fragments of 

 bone obtained by him twenty years ago from this same "Ossi- 

 ferous zone." Mr. Whitaker's specimen of Hyper odape Jon was 

 also obtained from the upper sandstone. — On the posterior por- 

 tion of a lower jaw of Labyrinthodon (L. lavisi) from the Trias 

 of Sidmouth, by Mr. Harry Govier Seeley, F.L.S., Professor 

 of Physical Geography in Bedford College, London. After 

 referring to the doubtful position of the Labyrinthodontia in the 

 system, and expressing his doubts as to the occurrence ot the 

 genus Mastodonsaiirus in Britain, the author proceeded to 

 describe in detail the posterior part of the right ramus of the 

 lower jaw of a Labyrinthodont, obtained by Mr. Lavis from the 

 ossiferous zone of the Trias near Sidmouth, the position of which 

 was described by that gentleman in the preceding paper. The 

 specimen, which is 13 inches long, and perfectly free from 

 matrix, shows that the lower jaw in Labyrinthodonts not only 

 contains articular, angular, and dentary elements, as hitherto 

 supposed, but also separate sphenial and surangtilar elements, 

 and probably a distinct coronoid bone. These bones were de- 

 scribed in aetail, and the author remarked that although they 

 are somewhat reptilian in aspect and arrangement, they are not 

 very suggestive as to the affinities of Labyrinthodon. They sur- 

 round a central hollow space, which no doubt received the 

 primitive cartilage round which the bones were ossified ; and the 

 persistence of this character would seem to be a link rather with 

 the lower than with the higher vertebrata. The jaw differs from 

 the Batrachian mandible in possessing well-developed angular and 

 surangular elements, and some reptiles, such as crocodiles and 

 the marine Chelonia, present analogies in the perforations, the 

 structure of the jaw, and the sculpture of the bones. In size 

 the specimen is almost identical with that figured by Mr. Miall as 

 belonging to Labyrinthodon pachygnathus, but the depths and 

 outlines of the postarticular part of the jaw, and differences in 

 the sculpture of the lateral subarticular ornament, furnish dis- 

 tinctive characters which lead the author to describe the present 

 species as representing a new species, which he names, in honour 

 of its discoverer, Labyrinthodon Lavisi. The author briefly no- 

 ticed several other bones and fragments obtained by Mr. Lavis 

 in the same locality, some of wliich probably belonged to the 

 same skeleton. — On the discovery of Mclonites in Britain, by 

 Mr. Walter Keeping, communicated by Prof. T. M 'Kenny 

 Hughes. The author described a specimen from the carbo- 



