268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



of two sidereal systems interpenetrating one another — a very fas- 

 cinating hypothesis. 



This hypothesis of the two drifts deduced from a discussion of the 

 proper motions of the stars is strongly confirmed by Campbell's inves- 

 tigation of the radial velocities. We should expect, if there are two 

 drifts of stars toward and from this point in Orion, that the radial 

 velocities of stars in this and the opposite point of the sky should be 

 greater than at points at right angles to these directions. Campbell 

 found that the velocities in the vertex and antivertex are 33 per cent 

 greater than at points about 90° from these. 



Besides the two drifts of stars we have smaller groups of stars in 

 different regions of the sky, all the stars in a group having a motion 

 approximately in the same direction and of the same velocity. Such 

 groups are quite different from drifts where there is only a pre- 

 ponderance of motion in one direction, and are called star streams. 

 Perhaps an analogy may help to make the matter of star drifts, star 

 streams, and solar motion clearer. If you imagine yourself walking in 

 a park where there are many people moving about at random it is evi- 

 dent that, speaking generally, the space between those you are approach- 

 ing opens out, between those you are moving away from closes in, 

 while people at either side, on the whole, appear to move backward. 

 The apparent motion of the mass due to your own motion is analogous 

 to the star drift due to the solar motion. If among the people there 

 are, say, a company of soldiers or a picnic party moving in a given 

 direction we have an analogue of a star stream. 



It was first pointed out by R. A. Proctor, about 40 years ago, that 

 five of the stars of the Dipper have proper motions in the same direc- 

 tion and of approximately the same magnitude ; and this stream has 

 within the last two years been thoroughly investigated by Luden- 

 dorff, of Potsdam, who determined their radial velocities, and more 

 recently by Herzsprung, who found that the stars Sirius and a Coronse 

 Borealis, as well as some fainter stars, also belong to the group. It is 

 a comparatively simple problem mathematically to determine the 

 mean parallax or distance of such a stream when we know the con- 

 vergent point or apex and the proper motions of the group with the 

 radial velocities of two or three of them. The parallax of the five 

 stars of the Dipper comes out as .0352", which is equivalent to a light 

 journey of about 90 years, while they are all moving at a velocity of 

 20 kilometers per second toward a point in the southern part of the 

 constellation Sagittarius (a = 303.2°, d= -36.6°). It is shown 

 further that the stars of this group are at the same order of distance 

 apart as the sun is from some of the nearest stars — about 20 or 30 

 light years — and that they are about 100 times as bright as the sun. 



Another group or star stream has recently been found in the con- 

 stellation Taurus by Prof. Boss, consisting of 39 stars, forming a 



