34 § Dr. Herschel's Account of the Changes that have happened 
the angle of position. In the year 1779, it was 32 0 47' north 
preceding; and, by a mean of the three last measures I have 
taken, it is now only io° 53'. In the space of about 23 years 
and a half, therefore, the angle of position has manifestly under- 
gone a diminution, of no less than 21 0 54' ; and, that this change 
has been brought on by a regular and gradual decrease of the 
angle, will be seen when the rest of the measures come to be 
examined. 
The accuracy of the micrometer which has been used, when 
the angles of position were taken, being of the utmost im- 
portance, it becomes necessary to ascertain how far it will be 
safe to rely on the result of the measures. It might be easily 
shown that, in the day time, a given angle, delineated on a card, 
and stuck up at a convenient distance, may be full as accurately 
measured by a telescope furnished with this micrometer, as it 
can be done by any known method, when the card is laid on a 
table before us ; but this would not answer my purpose. For, 
objects in motion, like the stars, especially when at a distance 
from the pole, cannot be measured with such steadiness as those 
which are near us, and at rest. The method of illuminating the 
wires, and other circumstances, will likewise affect the accuracy 
of the angles that are measured, especially when the distance of 
the stars is very small. I shall therefore have recourse to astro- 
nomical observations, in order to see what the micrometer has 
actually done. 
January 22, 1802. The position of A Orionis was taken. 
1st measure, 52 0 38' south preceding; 2d measure, 54 0 14'. 
Mean of the two measures, 53 0 2 6'. Deviation of the measures 
from the mean, 48'. 
March 4, 1802. 11 Monocerotis. 1st measure, 28° 18' south 
