720 Beviews. [Oct., 



He therefore marks down its place, and again and again directs Lis 

 gaze towards it, until before long lie fancies that it does not occupy- 

 quite the same relative position as it did when he first noticed it. 

 With a curiosity thus excited he patiently continues his examination, 

 until at length, to his inexpressible joy, the wandering nature of the 

 strange star becomes a matter of certainty. He then traces its apparent 

 course along the zodiac and begins to reason on the data ascertained, 

 and by a variety of simple processes, which even the unscientific reader 

 cannot fail to follow and comprehend (especially as they are illustrated 

 by admirably-executed diagrams and star maps), determines approxi- 

 mately its distance, velocity, and period. Our limits forbid anything 

 like a detailed examination of these processes. We may say, however, 

 that the entire mathematical treatment of the subject is unusually clear 

 and forcible. So soon, however, as the author gets beyond the region 

 of demonstration, he appears to us to lay aside his usual clear-sighted- 

 ness, and often to see things as through a haze, his reasoning from 

 evidence which is only probable appearing much less forcible than it 

 is when that evidence is demonstrative. 



Before, however, noticing any of those matters on which we are 

 compelled to differ from the author, we must call attention to one 

 feature of the work which in our opinion greatly enhances its value. 

 In the second appendix are a number of important tables, in each of 

 which two values are given for every element whose determination 

 depends on the sun's distance. The first values, which are the ones 

 usually given in astronomical works, are those deduced from the solar 

 parallax 8""5776 given by Encke. But more modern researches have 

 tended to show that this is too small, and that consequently the sun's 

 distance, which depends on it, as well as the distances of all bodies 

 which are estimated in terms of the sun's distance, are too great. 

 Since this was first pointed out by M. Hansen it has been confirmed 

 by two entirely independent methods, viz. by M. Foucault's determina- 

 tion that the velocity of light has hitherto been overestimated, the 

 true velocity making the sun's distance correspond with the suggested 

 alteration, and by the determination as the result of a system of 

 observation set on foot by Dr. Winnecke, Vice-Director of the 

 Observatory at Pulkowa, that 15", the received value of the horizontal 

 parallax of Mars when in opposition, is too small by about T J T part, thus 

 giving a corrected value, from which likewise the old estimate of the 

 sun's distance from the earth is shown to be too great by about the same 

 quantity as by the other plans. With this newly suggested and highly 

 probable estimate the second values in Mr. Proctor's tables correspond. 



Though not mentioned in the title-page, some space is devoted to 

 a rapid sketch of the progress of astronomy from very early to very 

 recent times. Such a sketch, however interesting in itself, would, 

 perhaps, have been hardly relevant, had not each great discovery as it 

 is described been applied by the author to the illustration of some 

 point with reference to Saturn. It is a curious circumstance, and one 

 not noticed by Mr. Proctor, although in the shape of footnotes much 

 curious general information is thrown in, that Copernicus, the origi- 

 nator of one of the greatest revolutions in modern science, a revolution 



