ASTRONOMY. 305 



the existence or non-existence of Yulcan. It has simply been shown 

 that this device is not suitable for detecting such a planet. 



"I may mention that, during the summer of 1881, this same experi- 

 ment was tried by Dr. Langley, with far more satisfactory arrangements 

 than those of the solar observatory, and that it was a failure with him 

 •also, and that Professor Harrington, of Ann Arbor Observatory, also 

 failed to find any material advantage with a (horizontal) tube 150 feet 

 long." 



/', 



The Earth. — We extract from the Academy the following review (by 

 G. F. Eodwell) of an important book entitled The Physics of the Earth'' s 

 Crust,, by the Eev. Osmond Fisher: 



^'The author in twenty-one chajiters discusses the principal facts con- 

 nected with the interior heat of the earth, the elevations and depres- 

 sions of its surface, and the causes and effects of volcanic action. He 

 shows that the rate of increase of temperature, as the distance beneath 

 the earth's surface is augmented, is, on the whole, an equable one, and 

 may be taken to average about 1^ F. for every fifty-one feet (misprinted 

 degrees, p. 267) ot descent. And thus at a depth of about thirty miles 

 all known rocks would be in a state of fusion. As to the condition of 

 the interior of the earth, we are first led to a discussion of the density. 

 The surface density is between 2J.56 and 2.75, while the mean density 

 of the whole earth is 5.5. Thus the density considerably increases as 

 we approach the center of the earth. Everything points to the conclu- 

 sion that the earth has once been in a molten condition ; the main ques- 

 tion for consideration is whether it is still molten within, or whether 

 this condition has passed away, and it is now solid. It has been thought 

 by some, however, that the interior of the earth may be 'potentially hot' 

 — that is to say, really solid, on account of the enormous pressure to 

 which it is subjected, but ready to become fluid at any moment when 

 the pressure is diminished or removed. Having discussed the argu- 

 ments of Hopkins and of Sir William Thomson, the author asserts that 

 the requisite great rigidity which the earth must possess in order to en- 

 able it to resist the deforming influence of the attraction of the sun and 

 moon does not require that the earth should be absolutely solid from the 

 center to the circumference. A rigid nucleus nearly approaching the 

 size of the whole globe, covered by a fluid substratum of no great thick- 

 ness in comparison with the radius, with an outer crust of less density 

 floating upon it, would meet the difliculty. 'This is the supi^osition,' 

 says the author, ' as to the condition of the earth, which appears, on 

 the whole, to satisfy best the requirements both of geology and of phys- 

 ics.' Thus the solid nucleus would owe its solidity to the great super- 

 incumbent pressure, while the outer crust would owe its solidity to hav- 

 ing become cool through radiation, while the fluid substratum would re- 

 main in that condition because it would not be submitted to sufficient 

 pressure to render it solid, while it would retain sufficient heat to render 

 it molten. As to the density, von Waltershausen has calculated that 

 H. Mis. 26 20 



