SECTION III.-ANGULAR MEASURES, ASTRONOMICAL. TIME OBSERVATIONS. ALL these observations were, with one or two exceptions, taken with the sextant-horizon; on account of the portability and expedition of that instrument: but its accuracy was usually considered somewhere, not nearer than 2′ or 3′; though by care it might be brought within 1'. Latitude observations by various stars were occasionally taken as a check on the index-error; and though the time was usually obtained from the sun, it was also sometimes found from star observations; which last occasions may be distinguished in the following table, by the hours against which the quantity for the day is entered. The chronometer alluded to was my pocket-watch, compensated for temperature, and going, as will be observed from the column of error on mean solar time, very fairly,-considering the amount of motion VOL. II. M it was subjected to. The first column of errors on apparent solar time, was computed for the sake of having that sort of time certain for the meteorological observations; and the last, or the errors on sidereal time, for the sake of the astronomical observations with the Playfair alt-azimuth instrument ; which, however, being only employed on the Polestar, never required the time to be known with any very great degree of exactness. In the first fortnight following January 24th, there was only one day on which weather prevented me from obtaining a time-observation of the sun; and the climate was so fine, that the series could have been kept up, probably for the whole four months, almost daily; but my time was too much limited by other work, to allow of doing more than what is given below : LATITUDE OBSERVATIONS. MARCH 11-APRIL 10. WITH THE PLAYFAIR ALT-AZIMUTH INSTRUMENT, MOUNTED ON A FIRM TRIPOD. On March 11, at East Tombs,-Barometer = 30-06, and Thermo68.6 at 14h. 0m. per watch,—the following observations were taken for Latitude :— meter On March 18, at East Tombs,-Barometer = 30.20, and 12 51 20 9 21 14 29 14 4-1′ 44′′ + 0° 46′ 23′′ 29° 58′ 43′′ The meridional distance between the parallels of East Tombs and the centre of Great Pyramid, being paced,-was found equal nearly to 10,000 inches ; wherefore a correction of +8′′ is necessary to reduce the above latitude of East Tombs to that of Great Pyramid, Great Pyramid being north of East Tombs. |
