88 PHILOSOPHICAL TRANSACTIONS. [ANNO 1797. 



appears to be at a stand, and not postpone this practice till an indolent unhealthy 

 state takes place, which too often terminates in opacities no applications can after- 

 wards remove. 



11. Observations on Horizontal Refractions which affect the appearance of Terres- 

 trial Objects, and the Dip, or Depression of the Horizon of the Sea. By Jos. 

 Huddart, Esq., F. R. S. p. 2Q. 



The variation and uncertainty of the dip, in different states of the air, taken at 

 the same altitude above the level of the sea, was the occasion of turning my thoughts 

 to this subject; as it renders the observed latitude incorrect, by giving an erroneous 

 zenith distance of a celestial object. I have often observed that low lands and the 

 extremity of head lands or points, forming an acute angle with the horizon of the 

 sea, and viewed from a distance beyond it, appear elevated above it, with an open 

 space between the land and the sea. The most remarkable instance of this appear- 

 ance of the land I observed at Macao, for several days previous to a typhoon, in 

 which the Locko lost her topmasts in Macao roads ; the points of the islands and 

 low lands appearing the highest, and the spaces between them and the sea the 

 largest, I ever saw. I believe it arises, and is proportional to the evaporation going 

 on from the sea; and in reflecting on this phenomenon, I am convinced that those 

 appearances must arise from refraction, and that instead of the density of the at- 

 mosphere increasing to the surface of the sea, it must decrease from some space 

 above it; and that evaporation is the principal cause which prevents the uniformity 

 of density and refraction being continued, by the general law, down to the surface 

 of the earth : and I am inclined to believe, though I mention it here as a conjecture, 

 that the difference of specific gravity in the particles of the atmosphere may be a 

 principal agent in evaporation; for the corpuscles of air, from their affinity with 

 water, being combined at the surface of the fluid from expansion, form air spe- 

 cifically lighter than the drier atmosphere; and therefore float, or rise, from that 

 principle, as steam from water; and in their rising (the surrounding corpuscles 

 from the same cause imbibing a part of the moisture,) become continually drier as 

 they ascend, yet continue ascending till they become equally dense with the air*. 

 However, these conjectures I shall leave, and proceed to the following observations 

 on refractions. • 



In the year 17 93, when at Allonby, in Cumberland, I made some remarks on 

 the appearance of the Abbey Head, in Galloway, which is at about 7 leagues dis- 

 tance from Allonby ; and from my window, at 50 feet above the level of the sea 

 at that time of tide, I observed the appearance of the land about the head as re- 

 presented in pi. 2, fig. 1. There was a dry sand, xy, called Robin Rigg, between 



* Mr. Hamilton, in his very curious Essay on the Ascent of Vapours, does not allow of this prin- 

 ciple, even as an assistant} though by a remark (page 15) he takes notice of those appearances in the 

 horizon of the sea, and says they arise from a strong or unusual degree of refraction ; the contrary of 

 which I hope to illustrate in the course of this paper. — Orig. 





