3 ° 
Mr. Huddart's Observations 
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 af- 
finity with water, being combined at the surface of the fluid 
from expansion, form air specifically lighter than the drier at- 
mosphere ; and therefore float, or rise, from that principle, as 
steam from water; and in their rising (the surrounding cor- 
puscles from the same cause imbibing a part of the moisture), 
become continually drier as they ascend, yet continue ascend- 
ing until they become equally dense with the air.* However, 
these conjectures I shall leave, and proceed to the following 
observations upon refractions. 
In the year 1793, when at Allonby, in Cumberland, I made 
some remarks on the appearance of the Abbey Head, in Gallo- 
way, which in distance from Allonby is about seven leagues ; 
and from my window, at fifty feet above the level of the sea 
at that time of tide, I observed the appearance of the land 
about the Head as represented in Tab. I. fig. 1. There was 
a dry sand, xy, called Robin Rigg, between me and the Head, 
at the distance from my house of between three and four miles, 
over which I saw the horizon of the sea, H O ; the sand at this 
time was about three or four feet above the level of the sea. 
* Mr. Hamilton, in his very curious Essay on the Ascent of Vapours, does not 
allow of this principle, 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. 
