92 Mr. David Gill [May 23, 



facts, facts that have contributed more to the advancement of sidereal 

 astronomy than all the speculations of preceding centuries. They 

 point to us the lesson that " art is long and life is short," that human 

 knowledge, in the slow developing phenomena of sidereal astronomy, 

 must be content to progress by the accumulating labours of successive 

 generations of men, that progress will be measured for generations 

 yet to come more by the amount of honest, well-directed and systema- 

 tically-discussed observation than by the most brilliant speculation, 

 and that in observation concentrated systematic effort on a special 

 thoughtfully-selected problem will be of more avail than the most 

 brilliant but disconnected work. 



I hope that no one present thinks from what I have said that I 

 undervalue the imaginative fervid mind that longs for the truth, and 

 whose fancy delights to speculate on these great subjects. On the 

 contrary, I think and I believe that without that fervid mind, with- 

 out that longing for the truth, no man is fitted for the work required 

 of him in such a field — for it is such a mind and such desires that 

 alone can sweeten the long watches of the night, and transform such 

 work from drudgery into a noble labour of love. 



It is for like reasons that I ask you to leave with me the 

 captivating realms of fancy this evening, and to enter the more 

 substantial realms of fact. 



We suppose ourselves, then, face to face with all the problems of 

 sidereal astronomy to which I have hastily referred — the human mind 

 is lost in speculation, and we are anxious to establish a solid 

 groundwork of fact. 



Now what in such circumstances would be the instinct of the 

 scientific mind ? 



The answer is unquestionable — viz. to measure — and no sooner 

 were astronomical instruments made of reasonable exactness than 

 astronomers did begin to measure, and to ask, are the distances of the 

 fixed stars measurable ? 



I should like to have given a short history of the early attempts 

 of astronomers to measure the distance of a fixed star. But I must 

 come at once to the time when the long baffled labours of astronomers 

 began to be crowned with success. 



Before I begin, it will save both time and circumlocution if I 

 define a word that we must frequently use — viz. the word " parallax." 

 It may be defined as the change in the apparent place of a star 

 produced by viewing it from a point other than that of reference. 

 [The lecturer here gave some practical illustrations of parallax.] 

 Our point of reference for stars is the sun, and as we view the stars 

 now from one side of the sun, and six mouths afterwards from a point 

 on the opposite side of the sun — that is, from two points 186 millions 

 of miles apart — we might expect to find a considerable change in 

 their apparent places. 



But previous to 1832 astronomers could not discover with any 

 certainty that such changes were sensible — or, putting it another way, 

 the stars were so distant that the diameter of the earth's orbit viewed 



