22 Memoir on the Diminulion of the 
diminishes it to 36° 21’ 30’, the sun then being in the 
first degree of Capricorn. I have taken this altitude with 
all thé aceuracy possible; I have also found for the altitude, 
corrected by the parallax in the beginning of Cancer, and 
‘ when at its maximum, 83° 31’ 30’. Subtracting from the 
greater the less of the two altitudes, we have 47° 10, the» 
half of which, or greater declination, is 23° 35’, which is 
what | have adopted in this Table. I have also compared 
a great many times the meridian altitudes at the beginning 
of Cancer and Capricorn, with the corresponding altitudes 
before and aticr noon; T have found them to agree with” 
the greatest declination I have observed ; therefore, I can 
warrant iis exactness. 
«| have chosen those two points of the ecliptic for this 
research, because, if there was an error in the sun’s place of 
even scveral minutes, that would not produce any sensible 
difference, the change of declination being at that time very 
smiall.’” 
Though the author says that he has employed a different 
parallax of the sun from Ptolomy’s, which is not indicated 
in the part cf the work we are in possession of, every thing 
induces us to believe, however, that, the difference is yery 
inconsiderable. We may therefore adopt kere, without any 
sensible error, the parallax of Ptolomy to re-establish Ebn- 
Jounis’ observations to what his instrument bas given. This 
parallax is 2'51”, and becomes 2’ 18” at 36° 21' 30” of alti- 
tude; and thus the smallest meridian altitude observed by 
Ebn-Jounis was 36° 19/12”. Subtracting from it 1/19”, 
on account of refraction, and adding to it 7” for the paral- 
jax, such as is given by modern observation, we shall have 
26° 18'0" for the smallest real altitude of the sun. To 
correct likewise the observation of the greatest altitude, we 
must subtract 18” on account of the false parallax, and 7” 
for refraction: we must besides increase it one second on 
account of the true parallax, which gives $3° 31' 6” for the 
altitude thus corrected. Halving the difference of the two 
latitudes, we have 23° 36° 33” for the ecliptic’s obliquity at 
Ebn-Jounis’ time; that is to say, about the year 1000. 
Half their sum gives 30° 5’27” for the latitude of Cairo. 
This latitude has been found 30°_3’ £0” by the French 
astronomers in the house of the Tustitute, situated at avery 
small distance from the spot where it is presumed, with 
much likelihood, that the Arabian astranomer made his ob- 
servation. Making use of the French observation, and 
comparing it with the greatest altitude of the sun, as deter- 
mined by Ebn-Jounis, we shall obtain an obliquity more 
: exact, 
