On the Transit Instrument of the Cambridge Observatory; Being a Supplement to a Former Paper

VII. On the transit instrument of the Cambridge Observatory; being a Supplement to a former Paper. By Robert Woodhouse, Esq. Plumian Professor of Astronomy in the University of Cambridge. Read January 19, 1826. In the brief account of the transit instrument which I had the honour some time ago of presenting to the Royal Society, I stated the circumstance of the instrument's deviation from the meridian arising from the unequal expansion of its braces; but no instance was then given of the magnitude of such deviation. I now subjoin one. On the morning of Oct. 15, (civil reckoning) after observing the passage of Regulus, the southern shutters were accidentally left open, so that when I returned to observe the inferior culmination of the pole star, the sun was shining on the upper western brace, the object-glass of the instrument being towards the zenith. The effect of this was a retardation of more than 25 seconds in the star's passage, as will thus appear: | Rate of Clock | Oct. 14. | Oct. 15. | Oct. 16. | Oct. 17. &c. | |---------------|----------|----------|----------|-------------| | | 0° 59' 20" | 0° 59' 20" | 0° 59' 19.5" | 0° 59' 20" | | | 12 59 44 | 12 59 17 | 0 59 19.5 | 0 59 20 | | | Polaris. | Polaris S. P. | Polaris. | Polaris. | Reversed the axis. I now view, with great suspicion, all the observations of the sun's transits, which I observed previously to the detection of that source of inequality which is the subject of the present, and of my former communication.