250 Prof. Dean’s observations of the solar eclipse Sept. 1811. 
On Sept. 16th it was examined by 12 pairs of equal altitudes, and 
found to be one second too fast for apparent time. From this mean 
neither of the pairs deviated more than four seconds. On the day of 
the eclipse by the mean of 20 pairs, corrected as before, the clock 
was exactly as on the preceding day, one second too fast for appa- 
rent time. These observations were not quite so close as the former 
ones on account of the wind, but no pair deviated more than ten sec- 
onds, 20 pairs were within five, and 15 within three seconds of the 
mean. 
I observed the Seas through a spy-glass, having an achromatic 
object-glass, and magnifying 11 times. It was fitted up so as to have 
a horizontal and vertical motion, and was prepared with a piece of 
common window-glass, smoked to a proper degree of darkness. John 
Johnson, Esq. observed through a spy-glass, magnifying 13 times, 
fixed in the same manner. We placed the glasses in a south window 
close to the clock, and agreed that each should determine for himself 
the second when he first perceived the impression. At 33 minutes 
after noon I directed our assistant to begin and count the seconds aus 
dibly and distinctly from minute to minute, till we had both determin- 
ed the eclipse to have begun. Mr. Johnson spoke about fifteen sec- 
onds after I had discovered it. At the end we placed our glasses in 
a couple of the west windows, in a room adjoining the clock, and 
proceeded in the same manner. Deducting one second for the error 
of the clock, the apparent times were as follows ; 
Beginning of the eclipse J.Dean - 0 98 51 
J. Johnson O 88 39 
End of the eclipse - J, Dean - 3 43 11 
J.Johnson - 3 43 25 
By subtracting 5° 26” from the beginning, or 5’ 28” from the end of 
the eclipse, the mean times corresponding will be obtained. 
