1832.^] Observations of the Transit of Mercury. 409 



very fine 4-feet achromatic telescope, belonging to Lieut. Pemberton,of 

 four inches aperture, mounted equatorially, and provided with a deli- 

 cate wire micrometer. The power used was 60, and the darkened 

 glass was varied to accommodate the light to the eye — the clouds 

 were sometimes sufficient alone to act as a screen against the sun's 

 rays. 



For the time we were indebted to Mr. Gray, with whose astronomical 

 clock our chronometer was compared before and after the observa- 

 tion ; thus, 



At 9 h. 10 m. a. m, m% Sm 



Chronometer slow of Ellicott's Clock, — 15 46.2 



Deduct error of Clock by Me. So. Time, + 4 55.2 



Chron. slow of Mean Time, 10 51.0 

 m. s. 

 At 5 h. 45 m. P. M. Chron. slow, 15 50.0 

 Clock fast, (daily rate 0.2 gaining), -f- 4 55.2 



10 54.3 



Chron. 's loss in 8 h. 35 m ~ ~~~~~ 3.8 



Daily rate losing, -,.... 10.33 



Hourly rate 0.43 s. 



Correcting the times noted, therefore, the subjoined are the obser- 

 vations expressed in mean solar time. From the rapid motion of the 

 sun, we found it quite impossible to measure the distance of Mercury 

 from either the advancing or the following limb with the wire micro- 

 meter; the planet's distance, however, from the tangent of the sun's 

 northern limb, or at right angles to his motion, was taken at three dif- 

 ferent times, during the series of observations. There is, however, more 

 or less uncertainty in this mode of measurement, from the difficulty of 

 bringing one of the wires of the micrometer to coincide with the 

 sun's limb, as from the darkened eye-glass, the wire ceases to be 

 visible as soon as it is off the disc; the divided object glass microme- 

 ter is in this respect preferable. 



For the equatorial measurements, we had resort to the times of 

 transit of the sun's two limbs and of the planet past the two wires of 

 the micrometer ; repeating the sights as frequently as the weather 

 would permit, by moving the telescope a little in advance before each 

 series of readings commenced. There was a small spot on the sun 

 indistinctly defined towards the advancing limb, the transits of which 

 were also noted : the time occupied by the planet's passage of a wire 

 was about half a second, but as small reliance could be placed on the 

 measurement of such a space, it has been preferred to insert the mean 

 of the two readings, as the passage of his centre. His disc appeared as a 



2f2 



