ANNUAL OF SCIENTIFIC DISCOVERY. 



water, displace each other, the preceding film being conveyed away 

 by the adhesion of the water in radial lines, which produce motion by 

 reaction on the fragments of camphor, causing them to rotate in the 

 same manner as a Barker's mill. These jets or films of camphor can 

 be rendered visible by various means, as by fixing the camphor in 

 water, and dusting the surface lightly with lycopodium powder, when 

 a series of currents produced by the films will be made visible. The 

 motions of the fragments of camphor on water are greatly influenced 

 and complicated by their mutual attraction, and by the attraction of 

 the sides of the vessel. The film of camphor diffused over the sur- 

 face of the water is very volatile, disappearing as fast as it is formed, 

 chiefly into the air, only a very small portion being retained by the 

 water. Hence camphor wastes away much more quickly at the sur- 

 face of the water than in water alone, or in air alone, because at the 

 surface the film is being constantly formed at the expense of the cam- 

 phor, and is spread out to the united action of air and water. What- 

 ever interferes with evaporation lowers or arrests the motions of the 

 camphor and the allied phenomena ; so, on the contrary, whatever pro- 

 motes evaporation exalts these phenomena. Effects which are displayed 

 with great energy on a bright and sunny day, are produced either slug- 

 gishly or not at all on a wet, dull, or foggy one. A fixed oil forming a 

 film on water will displace the camphor-film, and so permanently arrest 

 the motions of the camphor ; but a volatile oil will only arrest the mo- 

 tions while it is present and undergoing evaporation. The motions of 

 camphor on the surface of water are increased by the action of the 

 vapor of benzole and some other volatile substances, such vapors 

 condensing in the liquid form on the camphor, and being then diffused 

 by the adhesion of the water. 



EXTENT OF THE EARTH'S ATMOSPHERE. 



At the British Association, 1862, Prof. Challis presented a paper on 

 the above subject, the object of which was to show that the earth's 

 atmosphere is of limited extent, and reasons were adduced, in the ab- 

 sence of data for calculating the exact height, for concluding that it 

 does not extend to the moon. It was argued, on the hypothesis of the 

 atomic constitution of bodies, that the upward resultant of the molecu- 

 lar forces on any atom, since it decreases as the height increases, must 

 eventually become just equal to the force of gravity, and that beyond 

 the height at which this equality is satisfied there can be no more 

 atoms, the atmosphere terminating in a small finite density. It has 

 been generally stated that the earth's atmosphere is about forty-five 

 miles high, but on no definite grounds, and the estimates of the height 

 have been very various. Against the opinion that it extends as far as 

 the moon, it was argued that, as the moon would, in that case, attach 

 to itself a considerable portion by its gravitation, which would necessa- 

 rily have some connection with the rest, there would be a continual 

 drag on the portion more immediately surrounding the earth, and 

 intermediately on the earth itself, which would, in some degree, retard 

 the rotation on its axis. Hence, if, as there is reason to suppose, the 

 rotation be strictly uniform, the earth's atmosphere cannot extend 

 to the moon. The author also stated that if by balloon ascents the 



