34 REPORT— 1873. 



one side and out of it at the otlier, the result being that each carries away so much 

 of the motion of the train, and therefore renders it difficult for the engine to drive 

 the train. Each individual comes to the ground with an immense forward impetus 

 and rubs along the ground till this is lost, in fact he carries with him so much 

 motion of the train and converts it into heat by friction against the ground. 



Now something similar to this must happen to a substance in visible motion in 

 an enclosure of constant temperature. The rays of light and heat will play very 

 much the same part as the waves of sound, or as the crowd of people in the above 

 illustration, at least if we except those which fall perpendicularly upon the surface 

 of the moving body. The moving body is like the train, and the rays of light and 

 heat are similar to individuals entering the train from a stratum of aether at rest, 

 and leaving the train into a stratum of sether at rest again, each probably transmu- 

 ting into heat a certain small portion of the visible motion of the train as it were 

 by a species of friction. Of course the intensity of such an influence would depend 

 upon the intensity of the rays of light and heat. Now it matters not what the 

 particular kind of motion be which constitutes this train, and we may assert that 

 all directed motion will suffer from such a cause, and possibly according to the 

 same laws. Visible motion, such as that of a rotating disk or of a meteor is of 

 course one form of such motion ; but a current of electricity or of heat may equally 

 represent some form of directed motion. In fine we may perhaps suppose that all 

 forms of directed motion are resisted by this peculiar influence, which evidently 

 depends upon what we may term the temperatm-e of the ajther, or at least upon the 

 intensity of those vibrations which the aether transmits. 



Astronomy. 



On the Importance and Necessity of continued Systematic Ohservations on the 

 Moon's Surface. By W. R. Biet, F.E.A.S. 



of a large 



q 



Note on the Proper Motions of Nehulce. 

 By William Htiggins, D.C.L., LL.D., F.B.S. 



There are three kinds of motion which we may expect to exist in a nebula, which, 

 if sufficiently rapid, might be detected by the spectroscope : — 



1. A motion of rotation in the case of the planetary nebulaB, which might be dis- 

 covered by placing the slit of the spectroscope on opposite limbs of the nebula. 



2. A motion of translation in the visual direction of some portions of the nebu- 

 lous matter within the nebula. Such motion might possibly oe detected by com- 



a spectroscope of sufficient dispersive power, the spectra of different parts 

 nebula such as that in Orion. 

 A motion of translation in space of the nebula in the line of sight. 



The observations to be described were undertaken with the view of searching for 

 this last kind of motion, namely that of the whole nebula in the line of sight. For 

 this purpose it is necessary to compare the lines of the nebula with those of a ter- 

 restrial substance which has been found to be in the nebula. Now the coincidence 

 of the third and fourth line of the nebular spectrum with lines of hydrogen was 

 available in the case of a few only of the brightest nebulfe. 



I had found that the apparent coincidence of the brightest line of the nebulae with 

 the brightest line in the spectrum of nitrogen was not maintained when a more 

 powerful spectroscope was used. The nebular line was then seen to be thin and 

 defined, while the line of nitrogen appeared double and each of its components 

 nebulous at the edges. The thin line of the nebula coincides very nearly with the 

 less refrangible of the two lines forming the double line of nitrogen. 



Fortunately I fovmd a line which appears under some conditions of the spark in 

 the spectrum of lead, which is single, defined, and occurs exactly at that part of 

 the spectrum. This line is represented in Thalen's map by a short line, to indicate 



