576 EEPORT— 1891. 



9. Report of the Committee on the Seasonol Variations in the Temperature 

 of Lakes, Rivers, and Estuaries. See Reports, p. 454. 



10. 0)1 the probable Nature of the Bright Strenhs on the Moon. 

 By Dr. Ralph Copeland, F.R.A.S., F.R.8.E. 



In this paper the author described the chief features of the bright lunar streaks, 

 especially their invisibility when the shadows of the mountains are most con- 

 spicuous, and their great prominence when the lunar shadows are imperceptible. 

 It was explained that the brifiht streaks demanded for their visibility not so 

 much a high angle of illumination, as a front illumination. In other words, thev 

 become visible when the light falls more or less closely iu the line of sight. 



If this condition is fulfilled the streaks come prominently mto view quite 

 regardless of the inclination of the surfaces on which they occur. The surfaces 

 indeed may make almost any angle, either with the line of sight or with the sun's 

 rays, provided they are at all turned towards the common direction of the spectator 

 and tlie sun. 



An important deduction from this fundamental fact is that each elementary 

 portion of the streak surface is of a form that is symmetrical to the spectator from 

 whatever point it is seen. The sphere alone appears to fulfil this condition ; hence 

 it may be assumed that the surface of the strealc material must be made up of a 

 large number of more or less complete spherical surfaces. These minute surfaces 

 may be either concave or convex. We may therefore regard the streaks as being 

 produced by a material pitted with minute cavities of s) herical figure, or strewn 

 over with minute solid spheres. In the latter case it is probable that the material 

 is more or less transparent, or at least ti-anslucent. 



To test this hypotliesis, a plaster model of the moon 22 inches in diameter was 

 made, on which the bright streaks are repi'esented by lines of minute spherules of 

 transparent glass attached to the surface. These possess in a marked degree the 

 desired property of remaining inconspicuous under cross light, while they flash 

 out brilliantly when lit up from the front. Although the spherules are but l-50th 

 to I-30th inch in diameter, they are still too large in propoi'tion to the model, and 

 therefore east perceptible shadows when they would otherwise be invisible. This 

 might have been largely avoided by the tedious process of partially imbedding 

 them in the model. The corresponding diameJer on the moon's surface would be 

 from 2 to 3.^ miles. 



When suitably illuminated the phases of the model were found, on photometric 

 examination, to follow a law not very unlike that of the lunar phases as derived 

 by Zcillner from his own observations, and those of Sir John Herscbel, the light of 

 the 'full moon' being nearly five times that at quadrature. Without streaks the 

 model closely agreed with Lambert's formula for a non-reflectiug sphere, for which 

 the full disc is 3' 141 6 times as bright as the half disc illuminated from the side. 

 The paper was illustrated by a diagram showing the relative brightness of the 

 phases as well as by the model and photographs of the same. The model, suitably 

 illuminated, was also e.xhibited at the evening conversazioni of the Association. 



TUESDAY, AUGUST 25. 

 The following Report and Papers were read : — 

 1. Report of the Committee on Electrical Standards. — See Reports, p. 152. 



2. The Causes of Variation of Clark Standard Cells.^ By J. Swixeurnf. 



The various parts of the cell are examined separately. Any zinc will do, if 

 amalgamated before use. The greatest variations are due to impurities, such as 



' Published in full iu Electrical Etmeiv.AxLgxxsi 28, 1891. 



