8 



NATURE 



{May 2, 1878 



second and third chapters the true or Copernican system 

 of the universe is described, the obliquity of the ecliptic, 

 the seasons, &c., according to this system, the Keplerian 

 laws of planetary motion, and progress from Kepler to 

 Newton. The latter of these chapters is devoted to 

 Newton's discovery of universal gravitation and con- 

 sequences flowing from it; the gravitation of small 

 masses, the figure and density of the earth, the tides, the 

 inequalities in the motions of the planets produced by 

 their mutual attraction, and the relation of the planets to 

 the stars. The three chapters form the first part of 

 the work, or " the history of the world historically 

 developed." 



The second part is deroted'to practical astronomy, the 

 telescope and the successive improvements and modi- 

 fications introduced in its construction and application 

 down to the present day. The cumbrous yet elaborate 

 form in which an instrument, powerful for its time, was 

 used in the middle of the seventeenth century, is well 

 illustrated by an engraving of the great telescope used by 

 Blanchini in the observations whereby he attempted to 

 determine the time of rotation of the planet Venus upon 

 its axis, one of the instruments constructed by the cele- 

 brated Campani, mounted in the grounds of the Bar- 

 barini Palace at Rome, and extracted from the historical 

 work "Hesperi et Phosphori Nova Phenomena." As 

 specimens of modern optical and mechanical achieve- 

 ment, we have illustrations and descriptions of the great 

 reflectors of the Earl of Rosse and Mr. Lassell, of the 

 Melbourne instrument and the new reflector in the 

 grounds of the Observatory of Paris. The great 

 refractor of the Naval Observatory of Washington is 

 represented in the frontispiece— a reduction from the 

 picture forming one of the series in the last volume of 

 Washington Astronomical Observations; the instrument 

 with which observations have been made that have afforded 

 us the first really satisfactory knowledge of the elements of 

 the orbits of the four satellites of Uranus and the satel- 

 lite of Neptune, and what is of still greater interest, the 

 instrument with which Prof. Asaph Hall has brought to 

 light the two minute satellites of Mars, a discovery justly 

 characterised by Leverrier as one of "the most im- 

 portant observations of modern astronomy.' ' The applica- 

 tion of the telescope to celestial measurement, the meri- 

 dian circle and its use, the determinations of time and of 

 terrestrial longitudes, are also considered, and the author 

 proceeds to treat of parallax in general, and in particular 

 of the investigation of the solar parallax, through the 

 intervention of the parallax of one of the planets Venus 

 and Mars when nearest to the earth. Prof. Newcomb 

 supplies a sketch of progress and results in this direction 

 from the first application of the method to the planet 

 Mars, on occasion of the French expedition in 167 1, 

 when Richer was sent out to the colony at Cayenne, in 

 South America, to secure observations of Mars, while 

 corresponding observations were made at the Observa- 

 tory of Paris, from the discussion of which observations 

 Cassini made what is usually given as the first reliable 

 approximation to the amount of the sun' s parallax, the 

 resulting value being 9"-5. The author, however, remarks 

 upon the determination made by Huyghens at the end of 

 his "Systema Saturnium," as the best of the seventeenth 

 century, the reason, as he states, of its being the best being 



' that it was not founded on any attempt to measure the 

 parallax itself, which was then really incapable of mea- 

 surement, but on the probable magnitude of the earth 

 as a planet. The idea of Huyghens was that the earth, 

 being a planet, its magnitude would probably be some- 

 where near that of the average of the two planets on 

 each side of it, viz., Venus and Mars. So, taking the 

 mean of the diameters of Venus and Mars, and supposing 

 this to represent the diameter of the earth, he found the 

 angle which the semi-diameter of the supposed earth, 

 would subtend from the sun, which would be the solar 

 parallax. By a fortunate accident. Prof. Newcomb 

 remarks, Huyghens' s estimate was nearer the truth than 

 any determinations made previous to the transit of Venus 

 in 1769, " his result'for the distance of the sun being 

 25,086 semi-diameters of the earth, or 99,000,000 of 

 miles." But it is to be noted that if Huyghens had used 

 the correct measures of Venus and Mars, he would have 

 been further from the truth ; his telescopes showing the 

 planets with diameters in excess of the true ones, " he 

 just hit the diameter of the earth, and reached the true 

 solution of the problem." This attempt of Huyghens to 

 ascertain the amount of solar parallax, is not often men- 

 tioned in our astronomical treatises. 



The section bearing upon investigations of the solar 

 parallax from transits) of Venus, though brief, contains 

 some interesting facts ; the proceedings of the American 

 expeditions for the observation of the transit of 1874, are 

 particularly noticed. It appears that the stations finally 

 occupied by the observers sent out from the United States 

 were, Wladiwostok, in Siberia, Pekin and Nagasaki, 

 Japan, in the northern hemisphere, and Kerguelen Island, 

 Hobart Town, and Campbelltown, Tasmania, Queens- 

 town, N.Z., and Chatham Island in the opposite one. 

 The American astronomers relied chiefly upon the 

 photographic method of observing the transit, as pos- 

 sessing obvious advantages over the old method of 

 noting the contacts, and the author describes and illus- 

 trates by a diagram, how by the use of a telescope of 

 great length— nearly forty feet— difficulties in the measure- 

 ment of the photographs were'sought to be obviated by the 

 French and American parties, as well as by Lord Lindsay. 

 With regard to the success attending the American expe- 

 ditions. Prof. Newcomb, (who it may be remarked was 

 one of the principal agents in the arrangement of their 

 equipment and plans of observation) states that the full 

 number of photographs expected was not obtained at any 

 station — but that the result taking the stations collec- 

 tively, was about half this number ; the British, French, 

 and Russian expeditions were about equally successful, 

 while the most fortunate, as regards weather, were the 

 German parties who were successful at all six of their 

 stations. He adverts to the amount of labour attending 

 the investigation and measurement of the photographs, 

 and with respect to the time when a final result may be 

 expected to be worked ^out from the observations of the 

 transit of 1874, having in view^the comparison of the 

 whole to ascertain how consistent they are with each 

 other, adds: "this cannot be done for several years," 

 yet that upon the question whether it is worth while to 

 send out parties to observe the transit of 1882, which 

 must soon be a subject of discussion among astronomers, 

 the answer must depend very largely on the success of the 



