162 Captain Owen on Circummeridian Altitudes. 
apart, until the sun falls to a similar altitude as that first taken, 
(presuming we may have altitudes on both sides of meridian, or 
of the greatest altitude,) the time, to minutes and tenths, must be 
noted at each altitude, as well as the altitude itself. 
4, When the sun is within reach of the sextant at both hori- 
zons, or his altitude more than 55°, alternate altitudes from each 
horizon should be taken, because by this the corrections for 
semi-diameter, dip, and instrumental error are avoided, the refrac- 
tion and parallax only remaining to be applied. The sets from 
each horizon to be noted separately. 
5. Mean the altitudes from each horizon separately, take the 
difference of these means, halve it, which will be the mean 
Zenith Distance. 
If altitudes have been taken from one horizon only, their mean 
will of course be the mean altitude. 
6. Now note all the times (from the time of apparent meridian 
passage, or the middle time by equal altitudes by watch, if any 
have been taken) in minutes and tenths only. 
7. Square these times, and carry them out conveniently. 
8. Collect these squares into one sum, and mean them by di- 
viding by the number of observations; this mean divided by 60 
will reduce it to miles and decimal parts, which, multiplied by a, 
as reserved, will give the reduction to the greatest altitude, or 
least Zenith Distance. 
Generally, when the observer has been stationary, and the ob- 
ject observed was the sun, near noon, and the latitude only be 
required, no further reduction to the meridian will be necessary 
than that to the greatest altitude. Example, for form: — 
