566 Notices of Memoirs—Edward A. Reeves— 
former, were of great importance, since it was no longer necessary to 
construct the enormous large arcs and circles which had hitherto 
been indispensable to give anything like accuracy. 
The magnetic compass not only made an enormous difference in 
navigation and exploration by sea, since it enabled the sailor to 
launch boldly out into the unknown oceans with confidence, but it 
soon began to leave its mark on land-surveying and geographical 
exploration. Much has been written on the invention of the compass, 
and many have been the disputes upon the subject, but it was 
certainly in use in Mediterranean countries of Europe as early as the 
twelfth and thirteenth centuries. The date when it was first used 
for land-surveying is not known exactly, but in Europe it was 
probably about the early part of the sixteenth century... . 
The surveying equipment of the pioneer explorer of early days, 
say, of from twenty to sixty years ago, usually consisted of a sextant 
and artificial horizon, a chronometer or watch, prismatic compass, 
boiling-point thermometers, and aneroid. With the sextant and 
artificial horizon the astronomical observations for latitude and 
longitude were taken, as well as those for finding the error of the 
compass. The route was plotted from the compass bearings and 
adjusted to the astronomically determined positions. The latitudes 
were usually from meridian altitudes of the sun or stars, and 
longitudes from the local mean time derived from altitudes east or 
west of the meridian, compared with the times shown by the 
chronometer, which was supposed to give Greenwich mean time. 
The sextant, in the hands of a practical observer, is capable of 
giving results in latitude to within 10” or 20”, provided it is in 
adjustment, but the difficulty is that the observer has no proper 
means of testing for centering and graduation errors. 
The great drawback to the sextant for survey work is that it is 
impossible to take accurate rounds of horizontal angles with it, 
since, unless the points are all on the same level, the angles must be 
too large. It is essentially a navigator’s instrument, and nowadays 
has been almost entirely superseded by the theodolite for land- 
surveying. 
As regards the longitude, the difficulty was always to obtain 
a steady rate for the chronometer, owing principally to the unavoid- 
able oscillations and concussions met with in transit. Formerly it 
was customary to observe lunar distances for getting the Greenwich 
mean time instead of trusting to the chronometers, but these, even 
with the utmost care, are very unsatisfactory. 
In more recent years the occultation of a star method of finding 
the Greenwich mean time superseded almost entirely the lunar 
distance, but all of these so-called ‘absolute’ methods of finding 
longitude are fast becoming out of date since the more general 
introduction of triangulation and wireless telegraphy. 
Heights of land were usually obtained by the boiling-point 
thermometer or aneroid. 
This, then, was the usual equipment of the pioneer. With such 
an outfit the greater part of the first mapping of Africa and other 
regions of the world was carried out, with results that were more or 
