EXPECTED METEORIC DISPLAY. 253 



comefs tail. As I have -said, Biela's comet is a case in point, and so 

 obviously in point that it is difficult to understand how any mathema- 

 tician could follow the history of the case without at once recognizing 

 the error which nevertheless has misled and still misleads Professor 

 Tait. That double comet, with its tails projecting from the sun, crossed 

 the earth's path in or about the first week of September, 1872, travel- 

 ing on a path slanted to the plane of the earth's orbit at an angle of 

 twelve and a half degrees, and with a velocity considerably exceeding 

 that of the earth in her orbit. Moving at this rate, and with this in- 

 clination, the companion comets would of course attain in ten weeks a 

 position many millions of miles south of the plane of the earth's orbit. 

 Thus a line from the sun to either comet would not, where prolonged 

 into the tail, approach within many millions of miles of the earth's 

 orbit — that is, of any position which the earth can possibly occupy. 

 Both comets were even farther away from the actual position occupied 

 by the earth at the time when, nevertheless, astronomers predicted a 

 star-shower, and when, as they predicted, such a shower occurred. 

 For the comets had left that place ten or twelve weeks before, and 

 nearly the whole of the comets' motion had carried them away from that 

 place, whereas only a small part of their motion had carried them away 

 from the plane of the earth's orbit. In fact, no one who had studied 

 with any attention the circumstances of any predicted meteor-display 

 could have fallen into the mistake made by Professor Tait, a mistake 

 actually so elaborated as to be made the basis of an entirely novel, 

 and for other reasons utterly impossible, theory of comets' tails.* 



* I may here remark that the tone of the above paragraph is, in my opinion, altogether 

 objectionable, considered in itself. It is almost impossible even for the most careful 

 students of science to avoid making mistakes from time to time, and occasionally mistakes 

 of the most egregious nature. There is scarcely one of the great thinkers whose work 

 has most effectively advanced science, who has not made mistakes even in dealing with 

 his own special subject ; while those who, like the Herschels, Humboldt, and others, have 

 dealt from time to time with subjects outside their own labors, have naturally been ex- 

 posed to more serious misapprehensions. It is not wonderful that Professor Tait, engaged 

 chiefly in analytical and physical researches, should fall into errors in dealing with astro- 

 nomical matters, as when he discusses comets' tails, the solar corona, and so forth. But 

 such errors should be corrected genially and pleasantly, not sneeringly (which, indeed, I 

 have not done) nor censoriously. I must point out, however, that Professor Tait lays 

 himself open to the severer forms of correction by the perfect savagery of his own cor- 

 rections of mistakes made by those who chance to have offended him. The man who, 

 in his lecture on " Force," so fiercely denounced Tyndall for mere errors, or, rather, in- 

 exactnesses of verbiage which could mislead none ; the man who jeeringly exchiimed, 

 " These be thy gods, Israel," because one of the greatest physicists of the age omitted, 

 in defining work done in raising bodies, to mention that such bodies were on the earth, 

 not on Jupiter or elsewhere ; the man who has even honored me by his sneers at real mis- 

 takes of mine, and who with ingenious garbling has invented mistakes for me which I had 

 never made (apparently for no other reason than because I pleasantly expostulated with 

 him on one occasion for his attacks on Tyndall) — can hardly object to be corrected in the 

 hard though not harsh tone adopted above. If the tu quocpie defense be considered in- 

 sufficient, then let me note that Professor Tait, by advancing a theory capable of being 

 tested by evidence without being at the pains so to test it, and by refusing even to ex- 



