552

HISTOEY OF PHYSICAL ASTEONOMY.

existence of a sensible parallax in the star, the final result obtained by himbeing only a small fraction less than that deduced by Henderson. Fromobservations made at the Cape, during the years 1839-40, Mr. Maclear de-termined the parallax of the star to be OAO^, and from still more recentobservations, extending down to 1848, he has found it to be 0".9187. Thelatter may be considered as the definitive value of the parallax of the star.

The parallax of a. Centauri, as determined by the researches of Mr.Maclear, assigns to the star a distance from the earth which may be saidto amount, in round numbers, to 20 billions of miles. The successivepropagation of light supplies the only means of forming anything like anadequate conception of this immense distance. Now light traverses spaceat the rate of 192,000 miles in a second of time, and consequently thedistance traversed in a year would extend to 0.059 billions of miles.Hence it follows, that notwithstanding the amazing velocity of this agent,it would require about three years and a quarter to pass from a. Centaurito the earth. The distance of 61 Cygni is about 60 billions of miles;consequently the light of that star does not reach the earth in less thanten years after its emission !

Knowing the parallaxes of 61 Cygni and a Centauri, and also theirapparent proper motions, it is hence easy to deduce the absolute amountof space through which each of these stars is transported in the course of ayear. Thus, in the case of 61 Cygni, which has an annual proper motionof about 5", since the radius of the terrestrial orbit subtends only an angleof 0".348 when viewed at the distance of the star, it follows, that thespace through which the star is annually transported in virtue of itsproper motion, exceeds the radius of the terrestrial orbit in the proportionof 5" to 0"348. Now the radius of the terrestrial orbit may be esti-mated, in round numbers, at 95 millions of miles. Hence it may be ascer-tained, by a simple process of arithmetic, that the star, 01 Cygni, whoseproper motion can only be established by observations of the most delicatehind, and which in consequence was long regarded as a fixed star, iscontinually transported through space at the annual rate of 1333 millionsof miles! In the same way it may be shewn, that the linear spacethrough which a. Centauri is transported in the course of a year, amountsto no less than 371 millions of miles. It is manifest, from thesenumbers, that the epithet, fixed, can no longer be regarded as an appro-priate designation of any class of the celestial bodies.

Allusion has been made to M. Struve’s investigation of the parallax ofa Lyrse. The more recent researches of M. Peters, tend to prove thatthe star has a sensible parallax, although the result is considerably lessthan the parallax previously assigned to the star by M. Struve. As-suming the latter to be the true value, as being determined by a moretrustworthy method than that founded on observations of the zenithdistance of the star, which formed the groundwork of M. Peters’ re-searches, we may, by combining it with the light of the star as ascertainedby the experiments of Wollaston, arrive at an evaluation of the absolutemagnitude of the star. To this end it may he remarked that the parallax0".201, arrived at by M. Struve, supposes the distance of a Lyras fromthe earth to exceed the distance of the sun from the earth in the propor-tion of about 800,000 to 1. Now if the sun w r as transported to this distance,it would follow, from the diminution of light according to the reciprocalof the square of the distance, that the actual light of the sun would ex-ceed the light he would then emit, in the proportion of 640,000 millions