Similar observations were also made when the instruments were at Brill and Arbury Hill: they were as follows. Arbury Hill. July 11th, 1799. Watch regulated. Hours. Brill. Barome-Thermo- Remarks. ter. meter. In. pts. Degrees. 9A. M. 11 15 29,180 65,5 Light airs at SW. Cloudy, but sun shining now and then. 29,200 70,0 Ditto. Ditto. Cloudy. Brill on the Hill. July 11th, 1799. Watch regulated. 29,100 61,0 Light airs at SW. Appearances of rain from SW. Cloudy. Ditto. Clearer, but cloudy. Arbury Hill very distinct. Ditto. More cloudy and equally clear. Hours. Depressed. ter. meter. The air remarkably clear and free from tremor. Cloudy all Ditto Ditto ditto. More cloudy. 29,200 61,75 The lamp at Arbury H. very bright. Avery fine quiet night. The next opportunity which offered, was at the former station and Wendover: the observations were as follows. Arbury Hill. July 27th, 1799. Wendover. Barome-Thermo Watch regulated. Remarks. [and there. 28,728 62,0 Fresh wind from SW. Rather dark weather, sun shining here Hours. Depressed. ter. meter. In. pts. Degrees. 28,734 64,2 Ditto. 3456 12 10 28,738 63 5 Ditto. 4 12 22 28,740 64,0 Ditto. 11 50 28,740 64,2 Ditto. Hours. 12 17 28,740 61,0 Less wind, and the air very clear. Wendover perfectly distinct. Wendover. July 27th, 1799. Watch regulated. Arbury H. Barome-Thermo- ter. meter. In. pts Degrees Remarks. 29,030 53,2 Wind at SW, rather fresh; sun shining, and air very clear. 29,030 53,0 29,030 545 Less wind, and the air very steady. Arbury Hill very distinct. 29,100 54,0 Little wind. Dew falling very fast. Ditto. Another opportunity for making contemporary observations occurred, when the parties were on Broadway Beacon and Epwell: I place them last, because I think them inferior to the others. 29,100 60,5 Wind SW. Cloudy. Much rain preceding night... IP.M. 6 6 29,100 63,2 6 20 832 29,208 60,7 29,100 59,0 29,100 57,5 Ditto, but calmer; sun not shining at Broadway. Very calm, and cloudy all round. Ditto. Appearances of rain in SW quarter. Foggy, but easily perceive the tent at Broadway Beacon. Broadway Beacon. June 26th, 1799. Watch regulated. ! To determine the refractions on the first arc, White Horse and Whiteham Hills, we have the distance between those stations 88662,2 feet, which subtends an arc of 14' 32" nearly. To determine those on the second, we have the distance between Brill and Arbury Hill = 146530 feet, subtending an arc of 24′ 8′′,9: those on the third, Wendover and Arbury Hill, 210628 feet = 34′ 35′′; and, for finding the refractions from the two last tables, we have the distance from Broadway Beacon to Epwell = 80611,4 feet, which subtends an arc of 13′ 11′′ nearly. The depressions and elevations were all taken to the ground, excepting those which are marked with asterisks. At White Horse Hill and Whiteham Hill, lamps were used at the hours of 9 and 10: they were also made use of at Arbury Hill and Brill at 9 o'clock. In the first instances, the lamps were placed (the centres of them) 14 feet from the bottoms of the respective instruments; and in the last 24 feet. The height of the transit telescope above the ground was always 5 feet; therefore, an allowance must be made, at each station, for the angle which that space subtends at its corresponding one; this premised, the refraction will be found from one of the two following rules, viz. if A be the contained arc, and D d the observed depressions, the quantity answering to the refraction, R, will be expressed by A-D-d; or, if one of the angles should A+ed: these rules give the be an elevation, e, then R = refractions in the following table. 2 1. Arc. Refractions found from the preceding Angles of Elevation and Depression. White Horse Hill & Whiteham Hill. 4. Arc. Broadway Beacon and Epwell. Bb On examining the refractions obtained on the first arc, we perceive them to have been tolerably regular from 3 o'clock till 8; the mean being part of the contained arc. The height of Whiteham Hill is 576 feet, and that of White Horse Hill 893 feet, above the level of the sea: the ray passes, therefore, through a tract of air considerably elevated, as the country between the stations is, for the most part, flat and low. The air is not often clear enough, or sufficiently free from tremulous motions, for these delicate observations. On the present occasion, however, the state of it was highly fit for the purpose; and, as care was taken, I am of opinion an error of more than 3", taking that of the arch of altitude into the account, cannot have obtained in any of the angles. The refractions at 9 and 10 o'clock are less than at the preceding hours; but this does not appear to have been owing to any change in the refractive power of the air throughout the whole extent of the ray, because the depression of Whiteham Hill, from the other station, varied little at those hours. These changes in the observed angles of elevation at Whiteham, (44′′ and 42′′ being the differences,) without corresponding ones at White Horse Hill, prove that some partial alteration, from floating strata, had taken place in the refraction near the former station. Whoever considers the matter, must perceive a case may be constructed in which this will take place, causing a great variation in one of the angles, whilst the other apparently remains the same: and this suggested the idea, that to afford any accurate conclusions in this way, a long series of observations would be necessary. It furthermore appears, that dew could not have caused these differences at Whiteham Hill, since the same cause would equally operate to vary the observed angles at White Horse Hill; but those remained nearly the same. |