VV. On the Lines that divide each semidiurnal Are into Six 
equal Parts. By W. A. gpa Esa. F. R. S. Lonp. & 
En. 
i 
[Read 3d June 1816.] 
‘HE divisions of the day which different nations have em- 
i ployed, are denoted by hour-lines of various kinds on the 
‘sphere. Of these hour-lines, drawn on.a supposition that ne- 
glects the inequalities of the Earth’s motiGe, there are three 
Jinds. 
_ The first kind, denotes hours counted from nis sot equal 
to each other at all, declinations of the sun. ‘These lines are 
great circles on the sphere, passing through the poles of the 
equator, and every pair intercepting a similar arc on each of 
the parallels. Of this kind: are the hour-lines of sidereal time, 
counted from the meridian, and the hour-lines of solar time, 
counted from the meridian. 
The second kind of hour-line denotes hours counted from the 
horizon, equal to each other in duration at all declinations of the 
sun. These lines are great circles which touch the greatest vi- 
sible parallel on the one hand, and the greatest invisible paral- 
lel on the other; each pair of these great circles cuts off a si- 
milar are from si diurnal arc. They are the horizons of 
different 
