1909] on Recent Results of Astronomical Research. 571 



this way or following Bredicbiu's method, have generally given 

 repulsive forces according to his rule, i.e. not exceeding 18 times 

 the attractive force of the sun ; but there are two or three cases 

 where clear evidence has been attained of a repulsive force 36, and 

 in one case, namely, the case illustrated on the screen, Jaegermann 

 found a force of at any rate over 60 ; he put it at 89. 



These repulsive forces are determined from the motions of large 

 masses of cometary matter, or from the general direction of the stream- 

 ing of the tail. It is of interest to see whether the small knots or 

 bends or bright patches, which offer a definite mark for measurement, 

 show the same evidence of repulsion. It is necessary in pursuing 

 this inquiry to be very careful to choose for measurement something 

 that is a real material point. We are liable, for instance, to choose 

 as a mark the point of crossing of two. streamers ; that must be 

 avoided, for the speed of such a point is not generally that of the 

 material of the streamers. 



We have to bear in mind the distinction between the two causes 

 of motion, an initial velocity due to explosion of the nucleus, and 

 motion due to the action of a force ; a force of repulsion from the 

 sun produces not merely a velocity along the radius vector, but a 

 continually increasing velocity, that is to say, an acceleration. Now 

 one of the most important and perplexing results of studying the 

 present comet is that there is very little evidence of acceleration, at 

 least of the steady acceleration which would indicate a steady force. 

 The tendency seems to be for the velocity to increase rapidly at first, 

 but then to become constant. I showed just now two photographs 

 in which a detached mass had broken away from the comet ; it had 

 a velocity relative to the nucleus of 2' • 6 per hour in the interval 

 between those two photographs, shortly afterwards, according to 

 Professor Barnard's and our own photographs, it reached a speed 

 of 3' '4 per hour, and remained constant at that for three days. 

 There is a well-marked knot seen on photographs on October 3 

 which bears every indication of being a material feature, and not an 

 illusion due to the crossing of rays. This can be traced on our 

 photographs for 6 hours, during which time its distance from the 

 head (measured in the direction of the radius vector) increases from 

 50,000 miles to 200,000 miles. During the first hour there is just 

 a suspicion of an acceleration ; but afterwards it moved parallel to 

 the radius vector with a quite uniform velocity of 27,000 miles per 

 hour (nearly 8 miles a second). This is rather a good case, for there 

 are nine photographs in the series, and the point of measurement is 

 very well marked, but similar cases could be multiplied almost 

 indefinitely. As soon as the distance from the head has reached a 

 comparatively small limit, the repulsive force seems to cease. It 

 might be suggested that the uniformity of the velocity shows that it 

 was produced by the initial impulse, and that solar repulsion had 

 little to do with it. But I may remind you that the way in which 



