570 Scientific Intelligence. 



those which at first sight seem possible; for example, a planet 

 inside the orbit of Mercury or a swarm of small bodies near the 

 Sun. Bottlinger's hypothesis of the shading of gravitation by 

 interposing matter is attractive, but the curve he gets does not fit 

 the observed deviations and according to theory it should not be 

 able to account for them. The motion of the magnetic field of 

 the Earth has also been mentioned, but it is to be rejected because 

 this hypothesis would require changes in other parts of the 

 Moon's orbit which cannot be admitted. A diagram is given 

 which seems to indicate that there are changes in the motions of 

 the Earth and Mercury similar in their nature but smaller in mag- 

 nitude than those in the Moon. If this correlation turns out on 

 further examination to be real, it would appear that there is some 

 kind of a surge spreading through the solar system affecting 

 planets and satellites in the same way but to different degrees. 

 A suggestion that this surge is in some way connected with the 

 well-known changes in the frequency of the magnetic storms on 

 the Earth is brought forward by presenting in the diagram the 

 well-known sun-spot curve which follows closely the magnetic 

 storm-frequency curve. Professor Brown lays little stress on the 

 somewhat doubtful correlation. He believes, however, that it is 

 the only hypothesis which has some chance of receiving confirma- 

 tion or the reverse from sources other than that furnished by the 

 Moon's motion. 



Certain other results are obtained from an examination of the 

 observations of the Moon. In three directions the number 1/294 

 is obtained for the ellipticity of the Earth. This agrees with 

 Clarke's value obtained from geodetic measures (1/293-5) but 

 differs from Helmert's determination (1/298). Professor Brown 

 believes that the evidence for J/294 is sufficiently strong to 

 require its adoption for astronomical purposes. Finally it appears 

 from a series of terms in the motion that the inequalities of the 

 Moon's limb have a systematic character. Roughly speaking, the 

 edge of the southern half of the Moon is farther away from the 

 Moon's center of mass than the northern half. It is concluded 

 that the density of the crust of the former is less than that of the 

 latter in accordance with the principle of isostasy. The analogy 

 to the case of the Earth with its marked land and sea hemispheres 

 is pointed out. This result was obtained a short time previously 

 by Professor Bakhuysen from the examination of a smaller series 

 of observations. The larger quantity of evidence and the expla- 

 nation were obtained by Professor Brown independently. 



In the course of his address Professor Brown also gives a brief 

 description of the tables of the Moon's motion which are being 

 formed and makes a plea for the establishment at some govern- 

 ment observatory in the southern hemisphere of the Harvard 

 method for obtaining the Moon's position by photograph. 



An interesting statement as to the degree of accuracy of the 

 Newtonian law is made. From the agreement of the theoretical 

 and observed motions of the perigee it is concluded that the 

 index which the inverse square law contains does not differ from 

 2 by a fraction so great as 1/400,000,000. 



