7Q6 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



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 how- 

 ever, 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 cir- 

 cumstances ; but with the experience and continual practice of thus obtaining means 

 of computing these refractions, though we may not be able to determine the re- 

 fracting 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 entrusted 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 exten- 

 sion of the survey was an object of greater importance. I did not, however, lose 

 any opportunity which the subsequent season offered; the 1st was, when the in- 

 struments were at White Horse Hill and Whiteham Hill ; the 2d, when one was 

 stationed at Brill and the other at Arbury Hill; and the 3d opportunity offered it- 

 self, when one party was stationed at the latter place and the other at Wendover. 

 On these occasions, the instructions by which I governed myself were to observe 

 the elevation or depression of the corresponding station at the expiration of every 

 hour, beginning at 6 a.m. and to have the watch well regulated from observed al- 

 titudes of the sun's limb. I was also very minute in having entered in the book 

 the state of the weather ; to keep the instrument properly sheltered from the wind ; 



* As many instances of strong atmospherical refraction have been related, and ingeniously accounted 

 for, in some of the late publications of the r. s., 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 4, 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 

 its depression. 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 : he did so, 

 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 3(y.O" and the 

 last 30'.2". The unusual distinctness of this object led me to keep my eye a long time at the te- 

 lescope; and while my attention was engaged, 1 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 situa- 

 tion ; it then remained stationary, and as night drew on, the object became indistinct. The 

 following evening, I observed the depression again, and found it 2O/.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 before ob- 

 served, 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. — Orig. 



