164 Captain Owen on Circummeridian Altitudes. 
marking, that for common sea practice, whenever the object ob- 
served has a motion in altitude exceeding 3’ miles in a minute of 
time, it is favorable for equal altitudes, presuming the horizon to 
be good; and in low latitudes, or within the tropics, twenty 
minutes on each side the meridian has been found sufficient 
generally. This sort of practice shall now be shown to be avail- 
able to surveyors and geographers on shore, with a good common 
theodolite, so constructed that the telescope, having a diagonal 
eye-piece, may traverse through the zenith, clear of the azimuth 
circle, or horizontal plate and glass, or, at least, so as to observe 
the greatest altitudes required. 
Supposing an observer on shore to be furnished with such a 
theodolite as described, usually called Transit Theodolites, or 
azimuth and altitude instruments, by it alone he will be able to 
observe, under favorable circumstances, the latitude, by the use of 
azimuths, or theodolite bearings, instead of times by watch. 
Method of observing and noting Azimuths and Altitudes. 
The instrument being well set up, well adjusted, and well 
levelled, it is unimportant where the zero of the azimuth falls, 
because we want only the differences of azimuth for this sort of 
observation; but to make all reasonable use of the observations, 
not only for latitude, but for the variation of the needle, the first 
bearing taken should be for the direction of the needle from zero, 
or it will generally be more convenient to adjust the zero to the 
magnet, as commonly done. Then write in the field-book as 
follows : 
At , 20th May, 1844, about 114" A. M., set up theodo- 
lite, adjusted with zero to north magnetic, and obtained the fol- 
