426 TRANSACTIONS OF SECTION F. 



owing to its simplicity, soon superseded the former, were of great importance, 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 goon began to leave its mark on land- 

 surveying and geographical exploration. Much has been written on the inven- 

 tion 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. 



For the filling-in of the topographical features early forms of the plane- 

 table, or their prototypes the trigonometer and graphometer, came into use in 

 the sixteenth and seventeenth centuriee. Besides these the surveying per- 

 ambulator, much as is used at the present time, was a favourite instrument in 

 measuring distances along roads, and many of the road maps of England before 

 the Ordnance Survey were made by its means, combined with compass-bc-arings 

 and circumfactor angles. 



It is supposed that Ptolemy was fully alive to the fact that it was not 

 necessary to actually measure the whole length of an arc of the meridian, but 

 that some parts could be computed, or perhaps graphically obtained, much as is 

 now done in plane-tabling ; but, so far as we know, the first to introduce 

 triangulation from a measured base and angles was Willebrod Snell, a mathema- 

 tician of the Netherlands, who lived in the seventeenth century. The account 

 of his triangulation for obtaining the distance between Alkmaar and Bergen-op- 

 Zoom, in Holland, is well known, and it is not necessary for me to refer to it in 

 detail here; but its importance cannot be overestimated, since it laid the 

 foundation for all future work. Much has been done in later years, but this has 

 only meant the improvement of Snail's system, the perfecting of instruments 

 for the measurement of angles and bases, and more refinement in the com- 

 putations. 



Of all the instruments used by the surveyor, there is doubtless none more 

 important than the theodolite, which seems to have been first of all invented 

 by Leonard Digges. His invention is described in his book on surveying, which 

 was completed by his son and published in 1571. 



There is an interesting old theodolite of much the same design in Bleau'e 

 famous Dutch Atlas of the latter part of the eighteenth century. 



The ' common theodolite,' as it was called, since it had no telescope, carried 

 by Mason and Dixon to the United States, and used by them in their survey 

 of the boundary between Maryland and Pennsylvania in 1763-9, is now in the 

 R.G.S. Museum. It was made by Adams, of London, and was evidently only 

 intended for observing horizontal angles. It resembles what is generally known 

 as a circumferator more than a theodolite. The famous Ramsden theodolite, 

 which was used on the primary triangulation of the British Isles and later on 

 in India, has often been shown in books, and doubtless many of you are quite 

 familiar with its appearance. This has found a final resting-place in the 

 Ordnance Survey Office, Southampton. 



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 observa- 

 tion 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 astronomicallv 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, butthe difficulty 

 is that the observer has no proper means of testing for centering and graduation 

 frrors, 



