therefore, supposing that fall to be gradual and constant, all the way from the latter station to the surface of the sea at Bridgewater Bay, we shall get 24 feet, for the height of Lugshorn Corner from the surface of the sea. The altitude of this station, deduced from that of Trevose Head, is 49 feet; and, subtracting 3 feet from it, (the height of the bank on which the instrument stood above the moor,) we get 46 feet for the height of the moor at Lugshorn Corner, above the level of the sea at Bridgewater Bay. But this height, supposing the fall regular, is proved to be 24 feet. There is, therefore, a difference of 22 feet, granting the whole of this to be an error on the side of the survey: but, as the general surface of the moor at Bridgewater Bay is several feet above the surface of the sea, we may take a moiety of 24 feet, for the error of the computed height of the station at Lugshorn Corner. ART. XLVI. Matters relating to Refraction. The refractions contained in this account, like those in our former Papers, tend to prove, that when rays of light pass horizontally, and considerably distant from the surface of the earth, they are less bent or refracted from their rectilinear courses, than theory and opinion have laid down as fact. It is very certain, however, that objection lies against particular conclusions drawn from such data as we possess; because the angles of elevation and depression of corresponding stations are observed at different times, and almost always, therefore, under different circumstances; but, with the experience and continual practice of thus obtaining means of computing these refractions, although we may not be able to determine the refracting power of the air under given circumstances, yet, as the causes which render it variable, are as likely to predominate when the angles of depression or elevation are observed from low stations as when observed from high ones, we may be enabled to make some general deductions.* When the instrument formerly made use of by General Roy was intrusted to my care, I possessed the means of determining, in a more accurate manner than had yet been done, the refractive power of the air near the horizon. To devote much time to it, has not, as yet, been in my power; because a more rapid extension of the survey was an object of greater * As many instances of strong atmospherical refraction have been related, and ingeniously accounted for, in some of the late publications of the Royal Society, I think it right to mention, by way of note, a very extraordinary instance of its variability. In the month of June, 1795, when the instrument and party were stationed at Pilsden Hill, in Dorsetshire, on a particular day, at about the hour of four, I em ployed myself in observing the angles of depression or elevation of the surrounding hills. After I had done all that was necessary in this matter, I turned the telescope to Glastonbury Tor, and observed the depression of it. The air was so unusually clear, that, desirous of proving to a gentleman then with me in the observatory tent, the excellence of the telescope, I desired him to apply his eye to it: this he did, and, agreeably to a desire he expressed, I again took the depression of the upper part of the old building, which I was enabled to do with great accuracy, and found it 2" different; the first being 30,0", and the last 30',2". The unusual distinctness of this object, led me to keep my eye a long time at the telescope; and, whilst my attention was engaged, I perceived the top of the building gradually rise above the micrometer wire, and so continue to do, till it was elevated 10′,45′′ above its first apparent situation; it then remained stationary, and as night drew on, the object became indistinct. The following evening, I observed the depression again, and found it 29',50". To what cause this extraordinary change in the refraction could be owing, I am at a loss to conjecture. The former part of the day had been warm, with little wind, and cloudy. The thermometer, at the time of observation, was 65°, and continued stationary for a considerable time. The sky was cloudy, but yet, as I have before observed, the air was remarkably clear. The top of Glastonbury Tor, I suppose, is about 200 feet from the surface of Sedgemoor, over a considerable tract of which, the line joining Pilsden with that object passes. The gentleman of whom I speak, as being with me in the tent, was Captain DARCY, of the Royal Engineers, who, no doubt, well remembers the circumstance. importance. I did not, however, lose any opportunity which the subsequent season offered; the first was, when the instruments were at White Horse Hill and Whiteham Hill; the second, when one was stationed at Brill and the other at Arbury Hill; and the third opportunity offered itself, when one party was stationed at the latter place and the other at Wendover. On these occasions, the instructions which I communicated to Mr. WOOLCOT, and by which I governed myself, were to observe the elevation or depression of the corresponding station at the expiration of every hour, beginning at six A. M. and to have the watch well regulated from observed altitudes of the sun's limb. I requested him also to be very minute in entering on his book the state of the weather; to keep the instrument properly sheltered from the wind; to be always cautious to adjust his level; and also to insert the state of the air, as to temperature and density, by noting the thermometer and barometer. During the time we were at the two first stations, White Horse and Whiteham Hills, there was only one day when the air was sufficiently clear for the purpose; this was the 6th of June. On that day, the following observations were made at the same time as shewn by signal. In. pts. Degrees. 4 29,730 60,3 Light airs at SW. Sun not shining; remarkably clear. 624 29,724 62,5 6 14 6 10 29,728 58,7 29,732 58,5 6 11 ditto. ditto. ditto. ditto. 621 5 37 29,736 55,7 Ditto. Lamp at Shotover very bright. Dew falling Ditto. Ditto. 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. Remarks. Hours. Brill. Barome-Thermo- In. pts. Degrees. 9A.M. 29,180 65,5 Light airs at SW. Cloudy, but sun shining now and then. Ditto. Cloudy. 3 P.M. 6 29,162 68,0 4 10 5 29,168 72,5 9 10 30 29,13263,0 Ditto. Sun shining a little, yet free from any tremor. Hours. Brill on the Hill. July 11th, 1799. Watch regulated. 29,100 61,0 Light airs at SW. Appearances of rain from SW. Cloudy. 29,210 67,5 Ditto. Clearer, but cloudy. Arbury Hill very distinct. Ditto. More cloudy and equally clear. [round. 29,210 65,0 The air remarkably clear and free from tremor. Cloudy all 29,210 71,0 29,250 71,5 29,200 61,75 The lamp at Arbury H. very bright. A very 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. 5678 [and there. 28,728 62,0 Fresh wind from SW. Rather dark weather, sun shining here Hours. Depressed. ter. meter. In. pts Degrees. 12 II 28,740 64,0 Ditto. Hours. 12 17 28,740 61,0 Less wind, and the air very clear. Wendover perfectly distinct. Wendover. July 27th, 1799. Arbury H. Barome-Thermo- ter. meter. In. pts Degrees Watch regulated. '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. 19,100 60,5 Wind SW. Cloudy. Much rain preceding night. 29,208 60,7 29,100 59,0 Ditto. Appearances of rain in SW quarter. Foggy, but easily perceive the tent at Broadway Beacon.. Broadway Beacon. June 26th, 1799. Watch regulated. |