236' Dr. Herschel on the Direction of the . 
translation in space from theoretical reasons, can only be 
admitted as a very plausible hypothesis. 
It would be worth while for those who have fixed instru- 
ments, to strengthen this argument by observing the stars 
which are known to change their magnitudes periodically. 
For as we have great reason to ascribe these regular changes 
to a rotatory motion of the stars,* a real motion in space may 
be expected to attend it ; and the number of these stars is so 
considerable that their concurring testimony would be very 
desirable. 
Perhaps Algol, which according to these ideas must have a 
very quick rotatory motion, may be found to have also a con- 
siderable progressive one ; and if that should be ascertained, 
the position of the axis of the rotation of this star will be in a 
great measure thereby discovered. 
An argument from the real motion to a rotatory one is 
nearly of equal validity, and therefore all the stars that have a 
motion in space may be surmised to have also a rotation on 
their axes. 
Symptoms of parallactic Motions. 
But, setting aside theoretical arguments, I shall now proceed 
to such as may be drawn from observation ; and, as all paral- 
lactic motions are evident indications that the observer of them 
is not at rest, it will be necessary to explain three sorts of 
motions, of which the parallactic is one ; they will often engage 
our attention in the following discussion. 
Let the sun be supposed to move towards a certain part of 
the heavens, and since the whole solar system will have the 
* See Phil. Trans, for the year 1795, page 68. 
