[ 25 ] 
As to the particular nature and qualities of this 
luminous matter, we have been fometimes apt to 
imagine, that it cannot well be any very pon- 
derous fluid, but that it rather muft referable as to 
its confftence a very denfe and thick fog, which 
broods on the furface of the fun’s dark body. How 
far will this idea tend to facilitate our conceptions 
of the various phenomena of the fpots above de- 
fcribed ? 
It has been gathered from many obfervations,. 
that the time which the fpots take to traverfe the 
whole difc, is nearly equal to the time that they 
are hid by being on the oppofite furface. It is plain, 
that the time of their appearing upon the difc muft 
be fome fmall matter fhorter than that of their - 
being hid behind it, on account of our not feeing a 
complete hemifphere of the fun. But further, it 
muft now be conlidered, that when a fpot juft enters 
the difc, the part, which is firft vifible, is the fartheft 
umbra, by which time the fpot has really advanced 
a whole diameter of itfelf upon the dilc. And 
again, when the fame fpot goes off the difc,. it is 
evident, that the part, which is laft vifible, is then 
the fartheft umbra, on which account the con- 
tinuance of the fpot upon the difc will be ftior- 
tened by an interval of time, which correfponds 
nearly to the whole breadth of it. This, as well 
as the other appearances, defcribed in the firft part 
of this paper, concerning the change of the umbra 
and figure of the nucleus, when fpots approach 
the limb, are all well illuftrated, by making, in. 
a fphere, an excavation fimilar to what we have 
defcribed, the bottom of which may be painted 
black; 
