Mr Galbraith m a Table for correcting tlieSun^s Altitude. SI 7 
your way» In consequence of this^ I have been tempted to 
offer you the following small table, intended to facilitate the me- 
thod of correcting the observed altitude of the sun’s lower limb 
by the fore observation at sea. It contains the joint effect of 
the sun’s semi-diameter, dip of the horizon, refraction,, and pa^ 
rallax, and is always to be added to the observed altitude, to 
find the true. In the computation of the table, the sun’s semh 
diameter is taken at 16'; and its variation from this quan^ 
tity, for each month in the year, is given at the bottom of 
the table, which must he added to, or subtracted from, the 
correction found in the table, which may be done at sight, 
according as the sign is d- or — It may be remarked, too, 
that the table does not extend to altitudes lower than 5°, 
the lowest altitude at which observations can be taken to be 
depended on for their accuracy. This renders all the num- 
bers additive, except the small table of corrections for the varia- 
tion of the sun’s semi-diameter, which may, in most cases, urn 
less a dip-sector is used, be neglected ; since, according to the 
accurate observations with this instrument by the ingenious 
Captain Basil Hall, much greater errors, arising from the va- 
riable state of the horizontal refraction, when this instrument i$ 
not used, are unavoidable. 
EXAMPLE. 
The observed altitude of the sun’s lower limb by the fore obr. 
servation, is 48° 46', and height of the eye 20 feet, in the month 
,of November ; required the true altitude of the sun’s centre ? 
1. By this Table. % By the usual Tables. 
:Sun’s obs. alt. lower limb, 48° 46' - - 48° 46' 0 
Correction to 49®, 20 feet. Semi-diameter, -f 16 12 
and November, -f 11 Dip, - — 4 26 
— Refraction, — 50 
True alt. 43 57 Parallax, d- 6 
True alt. as before, 48 57 2 
Hence the sknplioity ,of our method is evident, while it ob^ 
viously possesses all the requisite accuracy, since it never can 
deviate more than one or two tenths of a minute from the trutb^ 
.and is therefore fully sufficient for all purposes at sea. 
JIdinburgh, Feb. 182Q. 
