THE SOLAR SYSTEM. 197 



which now surrounds the constant of aberration, but why ignore 

 the work of predecessors who were quite as able as ourselves ? If 

 it were desired to determine some one angle of a triangulation net 

 with special exactness, what would be thought of a man who 

 attempted to do so by repeated measurements of the angle in 

 question while he persistently neglected to adjust the net ? And 

 yet until recently astronomers have been doing precisely that kind 

 of thing with the solar parallax. 



I do not think there is any exaggeration in saying that the 

 trustworthy observations now on record for the determination of 

 the numerous quantities which are functions of the parallax could 

 not be duplicated by the most industrious astronomer w^orking 

 continuously for a thousand years. How, then, can w^e suppose 

 that the result properly deducible from them can be materially 

 affected by anything that any of us can do in a lifetime, unless we 

 are fortunate enough to invent methods of measurement vastly 

 superior to any hitherto imagined ? Probably the existing obser- 

 vations for the determination of most of these quantities are as 

 exact as any that can ever be made with our present instruments, 

 and if they were freed from constant errors they would certainly 

 give results very near the truth. To that end we have only to 

 form a system of simultaneous equations between all the observed 

 quantities, and then deduce the most probable values of these 

 quantities by the method of least squares. Perhaps some of you 

 may think that the value so obtained for the solar parallax would 

 depend largely upon the relative weights assigned to the various 

 quantities, but such is not the case. With almost any possible 

 system of weights the solar parallax will come out very nearly 

 8 "809" — 0*005 7", whence we have for the mean distance between 

 the earth and sun 92,797,000 miles with a probable error of only 

 59,700 miles ; and for the diameter of the solar system, measured 

 to its outermost member, the planet Neptune, 5,578,400,000 miles. 



It is the habit of centipedes to carry their young, clasped by 

 means of their legs, to all parts of the underside of the body, 

 though generally the young are clustered in dense masses. When 

 the young are thus bunched together, the body is coiled upon 

 itself at that part ; and the contrast between a centipede in this 

 position, says Mr. J. J. Quelch — who describes the centipede's 

 method in Nature — and a scorpion carrying her young upon her 

 back, just as a small oppossum does, is a marked one. 



