SOLAR MOTIOX. 237 



or approach to equality. We have said that the caku- investigation 

 lated velocities are such as would be true if the stars^wcre «^ f^ie propc 

 at the assumed distanccRj and if ilicir real motions were ^°^^^" ^ ^ ^ 

 performed in lines at right angles to the visual ray; id 

 ^hich it may here be objected that the last of these as- 

 Stimptions is so far from huviiip, any proof in its favour, 

 that even the highest probability is against it. We may 

 admit the truth of what the objection states, without ap- 

 prehending that any error could arise on that account, if 

 the solar motion were determined by this method. For 

 if the stars do not move at right anglers to the visual ray, 

 their real velocity will exceed the calculated one ; so that 

 in the first place we should certainly have the minimum 

 of their velocities : and if we were obliged, for want of 

 data, to leave the other limit of the motion unascertained, 

 it must be allowed to be a considerable point gained if 

 we could shew what is likely to be the least velocity of 

 the solar motion ; but a more satisfactory defence of the 

 method is, that if we were to assume a mean of all the 

 angular deviations from the perpendicular to the visual 

 ray that may take place in the directions of the real mo- 

 tions of the stars, the only position we could fix upon as 

 a mean would be an inclination of 45 degrees. For in 

 this case the cliance of a greater or smaller deviation 

 would be equal ; and when a number of stars are taken, 

 the deviations either way iuight then be supposed to com- 

 pensate each other ; but what is chiefly to our purpose, 

 not only the angle of 45 degrees, but also any other, 

 that might be fixed upon as a proper one to represent 

 the mean quantity of sideral motions, would lead exactly 

 to the same result of the solar velocity to be investigated. 

 For if t*e velocities of any two stars were equalized, ' 



when their motions are supposed to be perpendicular to 

 the visual ray, they would be as much so when they make 

 any other given angle with it ; and it is. the equalization 

 or approach to equality and not the quantity of the velo- 

 cities that is the spirit of this method, I have only to 

 add, that an equalization of the solar motion with that of 

 any star selected for the purpose may be had by a direct 

 method of calculation, and will therefore be of great use 

 in settling the rate of the motion to be determined, 



Vol. XV,— Nov. 180G. I i U 



