22 6 THE POPULAR SCIENCE MONTHLY. 



transit (to the whole earth), and again at its close, with the effects 

 of the earth's rotation in the interval. These diagrams are made 

 from those prepared by Mr. Proctor (to whose admirably lucid illus- 

 trations the writer is otherwise indebted), and by Mr. Hill, under the 

 charge of Prof. Coffin, of our own " Nautical Almanac" office ; and on 

 them have been marked the eight stations occupied by American par- 

 ties. The next transit, in December, 1882, will be visible, it may be 

 observed, from beginning to end, in the United States. 



On the whole, it will appear, from what has been stated, that a 

 transit of Venus, though not the only means of determining the sun's 

 distance, and not possessing the relative importance it once did, re- 

 mains probably the best, as it is the best known, and, if it may be so 

 called, the most classic method. 



Judging from what appears to be the probable error of our best 

 independent determinations of the solar parallax (those from Mars), 

 and the presumption that the majority of astronomers regard those 

 obtainable by modern methods from Venus as still better, it is no un- 

 reasonable anticipation that the probable error of the coming result 

 will not exceed one-hundredth of a second. In other words, it may be. 

 expected to be at least an even wager that the error of angular meas- 

 urement in the final result made up, let us remember, from the inde- 

 pendent results of observers working in distant parts of the globe 

 will not exceed that which would be represented by the breadth of a 

 hair, seen at the distance of one mile. So slight is that error, which 

 will seem so large when carried out in the enormous numbers which 

 represent the distance of the sun, and those numbers still more incon- 

 ceivable which represent his own distance from his brother stars. 



In one of the most remarkable writings which have descended to us 

 from ancient philosophy, the " Arenaria" of Archimedes, that geometer 

 undertakes to show his contemporaries that it is in the power of num- 

 ber to reckon not only every grain of sand upon the sea-shore or even 

 in the whole earth, but more than would fill a solid sphere extending 

 beyond the sun ; and, in the course of his demonstration, he describes 

 to us how he attempted to find its diameter by measurements carried 

 on with a staff and a rod, when the morning and evening mists ren- 

 dered its light bearable to the eye. If the striking picture of this 

 " Newton of the ancient world " gazing at the setting sun, to attempt, 

 with such rude means, a portion of the task which remains unsolved 

 after two thousand years, be recalled here, it is because it seems 

 fittingly to remind us of the early steps of that ascent on which man's 

 long effort has raised him to the power of questioning Nature with 

 means of the Avonderful exactness just described, and to remind us 

 also how long human thought has rested on the great problem to 

 which we hope this present month may bring an answer. 



