214 THE POPULAR SCIENCE MONTHLY. 



which cannot well be explained without them, and they offer far bet- 

 ter means of conveying a knowledge of the relative positions of the 

 various countries, seas, etc., than any maps. The great expense of 

 globes has hitherto prevented their very general use, but some experi- 

 ments are at present being made with a view to lessening the cost 

 of the construction, which it is hoped may be successful. I cannot 

 pass from this subject without alluding to that class of maps which 

 gives life to the large volumes of statistics which are accumulating 

 with such rapidity. On the Continent these maps are employed to an 

 extent unknown in this country, both for purposes of reference and 

 education, and they convey their information in a simple and effective 

 manner. 



-*- 



THE TRANSIT OF VEXUS. 



Bt Prof. S. P. LANGLEY, 



OF THE ALLEGHANY OBSERVATOBT. 



OX the 8th day of the present month, at a little before nine in the 

 evening of our time, the planet Venus will be first seen enter- 

 ing upon the face of the sun, from that side of the earth on which it 

 is then day, and to observe the event astronomers will have made their 

 way from all the principal countries of the civilized world. The spec- 

 tacle in itself offers nothing that is imposing ; to the naked eye, indeed, 

 nothing of it will be visible, and all that the best telescope can dis- 

 cern will be a small, black, circular spot moving across the upper part 

 of the solar disk, during some four and a half hours. The interest of 

 the occasion, as all know, lies in the rare opportunity it offers for ob- 

 taining the sun's distance from the earth ; but, as it is not so well 

 understood why this distance is wanted, why it has not been found 

 before, and what Venus has to do with determining it now, it is pro- 

 posed here to attempt to answer such questions, as fully as it can be 

 done in general and untechnical terms, and in a single article. 



The exact object to be obtained can be better understood after 

 considering what we know about the relations of the sun and planets, 

 and what we have yet to learn. We know already, then, with almost 

 entire exactness, the relative distances from the sun of every planet 

 (the earth included), so that, if we wished to make a map of the solar 

 system, on which the position of each member should be laid down 

 with great precision, we have already all the means at hand to do it. 

 Let us suppose such a map to be drawn, in which circles around a cen- 

 tral point represent the planetary orbits. Then the planets being 

 ranged in a line from the sun, and the distance of Venus from it being 

 let us say five inches, that of the earth will be seven, and that of 



