44 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



1869) refer to other series of experiments, as 

 follows : 



Some later observations have been made upon the 

 same subject in Paris, respectively by M. Bailie, at 

 the Ecole Polytechnique, and M. Marie-Davy at the 

 Paris Observatory. The former employed a concave 

 mirror of 39 centimetres aperture to condense the 

 moon's rays upon his pile, and also made us of a 

 Thomson's galvanometer. The one conclusion at 

 which he arrived was, that the full moon, at Paris 

 and in the summer months, gave as much heat to his 

 pile as a radiating surface 6.5 centimetres square, 

 maintained at boiling-water temperature and placed 

 at a distance of 35 metres. M. Marid-Davy has pub- 

 lished results of two series of measures secured dur- 

 ing the lunations of October and November last. 

 The first were made with a pile attached to a 9-inch 

 equatorial refractor, the second with an 8-inch mirror ; 

 the object-glass in the former case having been found 

 to intercept a large proportion of the heat-rays. M. 

 Marie-Davy's measures confirm those of Lord Bosse. 

 They show that the heating effect of the moon in- 

 creases with the illumination of the visible disk. 

 Between October 9, when the moon was four days 

 old, and October 20, when it was full, the measured 

 heat of the condensed beam of moonlight increased 

 from 0. 00017 (centigrade) to 0.00287. If this last 

 number be divided by the ratio of the area of the 

 concentrated image to the area of the object-glass, 

 we have twelve-millionths of a centigrade degree 

 as the direct heating power of the lull moon at 

 the earth's surface. This is the result given by 

 the object-glass ; that afforded by the mirror is 

 about six times as great. It will be seen that M. 

 Marid-Davy has converted his galvanometer indica- 

 tions into centigrade equivalents : how this conver- 

 sion was effected, and how the constancy of the scale 

 indications is secured, if it is secured, we are not in- 

 formed. He confirms Lord Eosse' s inference that 

 the proportion of solar to lunar radiation is about as 

 80,000 to 1, and likewise concludes that the moon 

 imparts to us no heat from an internal or cosmical 

 source. Further, he infers that the diffusive power 

 of the lunar surface is considerablej at least equal to 

 that of the least colored of terrestrial rocks : and he 

 finds that the lunar heat, by reason of its large per- 

 centage of obscure rays, is far more impressionable by 

 atmospheric humidity than that from the sun. 



It will be remembered that Prof. Smyth, in his 

 Teneriffe experiments, determined the heating power 

 of the full moon to be equal to one-third of that of a 

 Price's candle at a distance of 14 feet 9 inches. M. 

 Marid-Davy finds that such a candle at such a dis- 

 tance affects his pile to the extent of 0. 00075 centi- 

 grade, which he conceives to be the heating power 

 of the moon upon the summit of Teneriffe, upon the 

 supposition that the heat emitted by the respective 

 candle-flames was sensibly the same. 



Apparent Changes in the Moon. About the 

 middle of the year, Mr. Birt, who has devoted 

 much care to the observations of the moon, re- 

 ported that the number of spots seen at that 

 time on the floor of Plato was 35, 8 of which 

 had been detected since September 27, 1869. 

 Since then no less than 531 observations have 

 been made. The degree of visibility of eacli 

 spot is indicated in a table, regarding which 

 Mr. Birt says that "the number of spots in 

 which an increase of visibility has taken place 

 is nearly equal to that in which a decrease has 

 occurred, viz., 10 of the former and 11 of the 

 latter. Spot No. 3, a craterlet, has manifested 

 the greatest increase, and spot No. 19 the 

 greatest decrease. The extent of variation of 

 the separate spots is very irregular, and does 

 not seem to point to the operation of any gen- 



eral law. In one or two instances only have 

 neighboring spots been similarly affected." He 

 adds that "the observations of the twelve lu- 

 nations ending in March, 1870, extend consid- 

 erably the basis on which to form an intel- 

 ligible explanation of the phenomena ; it is, 

 nevertheless, too narrow to hazard more than 

 conjecture." 



Mean Motion of the Moon. In a paper read 

 before the National Academy of Sciences, in 

 April, Mr. Simon Newcomb discussed at some 

 length the problem of the apparent inequali- 

 ties of long period in the moon's mean motion, 

 and stated that it was really no nearer such a 

 solution as will agree with observation than 

 when it was left by Laplace. The author re- 

 marks that we have reason to suspect that the 

 motion of the crust of the earth is subject to 

 inequalities of an irregular character, which 

 in the present state of science can be detected 

 only by observations of the moon. He says : 

 " If we admit that the interior of the earth is 

 a fluid, and also admit that general changes in 

 the arrangement of this fluid are possible, we 

 have all that is necessary to account for con- 

 siderable changes in the rotation of the outer 

 crust. That this fluid, admitting its existence, 

 is not in a state of entire quiescence is rendered 

 probable by the phenomena of volcanoes and 

 earthquakes. If we suppose a large mass of 

 it to move from the equatorial regions to a po- 

 sition nearer the axis, a mass from the latter 

 position taking its place, the following effects 

 will follow : 



" 1. A diminution in the angular velocity of 

 the surface of the fluid, accompanied by a cor- 

 responding increase in the velocity of the axial 

 portion. The velocity of the outer crust will 

 then be gradually retarded by friction. 



" 2. The gradual transmission of the increased 

 rotation of the central mass to the surface by 

 friction and viscosity. The motion of the 

 crust will then be gradually accelerated. The 

 velocity of rotation finally attained will be 

 greater or less than the original velocity, ac- 

 cording as the radius of gyration of the fluid 

 mass is diminished or increased by the change 

 in the arrangement of the fluid. 



" * * * This suspicion can be neither con- 

 firmed nor removed until we have more posi- 

 tive knowledge than we now have of the pos- 

 sible inequalities which may be produced in 

 the mean motion of the moon by the action of 

 gravitation. 



"The operation of calculating these inequali- 

 ties, though complicated and difficult, is cer- 

 tainly within the powers of analysis. When 

 it is completely and thoroughly done, we may 

 ascertain whether the result can be made to 

 represent observations. If so, well ; the length 

 of the day is not variable, and the future po- 

 sitions of the moon can be safely predicted. 

 If not, it will follow either that the motion of 

 the moon is affected by other causes than the 

 gravitation of the known bodies of the solar 

 system, or the day is irregularly variable. 



