September 1, 1898.] 



KNOWLEDGE. 



171 



moon.'' This will, I trust, make it clear that the amount 

 of evaporation for a given rise of temperature is much 

 greater at the moon's surface than here, and it need hardly 

 be remarked that the difference of temperature between 

 the lunar midday and lunar midnight is probably much 

 greater than 30° Centigrade. But the white streaks do 

 not melt away or disappear during the lunar day. It is 

 therefore evident 

 that if they are 

 caused by a snow- 

 like deposit on either 

 side of volcanic 

 vents, the deposit 

 miist be thick enough 

 not to be melted 

 during theluuarday, 

 and yet the de- 

 posit must be thin 

 enough at its edges 

 to only partially 

 whiten the ground ; 

 but this is perhaps 

 possible on a rough 

 surface. I would 

 refer readers who 

 take an interest in 

 the theories as to 

 these bright streaks 

 to a paper published 

 in Knowledge for 

 May, 1890. 



Some of the dark 

 markings on the 

 moon are even more 

 mysterious than the 

 white ones. On ex- 

 amining our plates, 

 they might at first 

 sight be taken for 

 photographic de- 

 fects, but a close 

 comparison of two 

 photographs shows 

 that many of the 

 smallest dark marks 

 are exactly repeated, 

 and a comparison 

 with other photo- 

 graphs proves that 

 very little change 

 takes place in the 

 appearance of these 

 dark spots when the 

 sun is at different 

 altitudes. Take, for 

 example, the curious 

 dark spot on the 

 Mare Imbriumin the 

 fork of the ray be- 

 tween Eratosthenes 

 and Archimedes. It 

 looks like the half of 



* This relation between the force of grarity and the amount of 

 evaporation is probably the cause of the intense energy of the 

 eraporation phenomena which are observed when a comet approaches 

 the sun. The pressure of an atmosphere of vapoui-, about a small 

 body like a ewarm of stones, must be so small that the whole of any 

 volatile substance would, with a small increase of temperature, be 

 driven off before the pressure corresponding to saturation is reached. 



an elliptic crater surrounded by a black line, with an 

 intensely black spot at its southern end. If this marking, 

 as shown on Plates I. and II., is compared, very little 

 doubt will be entertained as to the real existence of its 

 smallest details. See also the two dark and two lighter 

 lines across the floor of the crater of Archimedes, and 

 examine the numerous small black tree-like markings on 



the southern and 

 eastern sides of its 

 elliptic ring ; they 

 look as if small 

 dark-coloured lava 

 streams had over- 

 flowed from the 

 crater and spread 

 out at its base. 



There are many 

 similar dark mark- 

 ings round other 

 craters — see, for ex- 

 ample, on Plate II. 

 the curious dark 

 tree-like structure 

 to the north-east of 

 Copernicus reach- 

 ing almost from the 

 rim of the crater to 

 the spurs of the 

 Carpathians ; and 

 there are other 

 somewhat similar 

 markings on a still 

 larger scale extend- 

 ing from the south- 

 ern rim of the crater 

 towards the south 

 and west, to a dis- 

 tance of nearly 

 double the diameter 

 of the crater. These 

 dark markings, un- 

 like the bright rays 

 which diverge from 

 Copernicus, spread 

 out at their ends, 

 and they are con- 

 nected with the re- 

 gion about the crater 

 by comparatively 

 narrow streams. 

 Whether they cor- 

 respond to streams 

 of dark-coloured lava 

 which once flowed 

 from the crater, or 

 to drainage areas 

 from the high ground 

 — they are on an 

 enormous scale com- 

 pared with similar 



Fig. 7. — A deeper-etched block from the same photograph as that reproduced terrestrial pheno- 



on the opposite page. mena. A river like 



I the Thames would appear as a very narrow meandering line 



beside them, so that it seems difficult to conceive of them 

 as the drainage areas from hot springs. But if hmar 

 eruptions are, like terrestrial eruptions, accompanied by a 

 great evolution of steam which rapidly falls as rain, thickly 

 charged with volcanic dust, vast rivers of mud may have 

 flowed down and discoloured the surface during the erup- 



