3S0 Dr. Mac Culloch on the Geology of Glen Tilt. 



In the ordinary operation of maritime surveying by the needle and 

 ■sights, the errors are often likely to be considerable ; more particu- 

 larly where some of the angles are determined with the instrument 

 placed on shore, mariners having been uninformed with respect to the 

 existence of such disturbing forces. To such causes it is very pro- 

 ble are owing the discrepancies found in charts of new or distant 

 coasts, where the instruments have even been good and the obser- 

 ver practically expert in its use. It is even probable that the charts 

 of the west coast of Scotland, constructed by Mackenzie with 

 abundant leisure and experience in his art, owe many of their very 

 glaring errors to this cause, which on many of these coasts is 

 exceedingly active. Want of care or want of accuracy, as far as this 

 could be obtained by the means which he possessed, cannot be im- 

 puted to him, as the soundings, distances, and enumeration of 

 even the most minute sunk rocks, is surprisingly accurate, render- 

 ing his work such, that even the most inexperienced pilot may enter 

 these intricate harbours and come to his anchorages writh no other 

 guide, as I have abundantly verified on numerous occasions. If, as 

 has been said, the well known map of North Wales constructed by 

 Evans, was also surveyed by the compass, we may perhaps attribute 

 a portion of the errors which it comprises to this neglected cause 

 among others. It is true that the repeated comparison of bearings 

 will afford some correction to inaccuracies of this nature, but a me- 

 dium of bad observations can never produce a good one. 



At any rate where such an instrument is to be used it will always 

 be prudent to erect it on a staiF at some distance from the ground, 

 as we shall then at least avoid those disturbances which result 

 from the contact, or very near approach of rocks when it is 

 placed on or near the ground. This addition is easy and of 

 little incumbrance to the operator, and as the compass and 



