272 Observations on Surveying Instruments, 



very likely to mislead those who pay any attention to them, 

 A simple fleur de lis on the north part of the needle, is all that 

 is necessary. 



With a view to the improvement of instruments, and to 

 obviate, if possible, some of the difficulties and embarrass- 

 ments which the practising surveyor has to encounter, I will 

 venture to add to the observations already made, a few 



Remarks on the errors of the Compass. 



Those in the use of ordinary instruments, will be many, 

 such as may arise from imperfect graduations, &c. ; but I 

 shall mention only a few of those which are common to all, 

 or at least the greater part. 



Most of the compasses that I have seen, have steel centre- 

 pins passing through the card, and extending from an inch, 

 to an inch and a half below, and frequently of the size of a 

 common crow's quill. 



Should this pin by any means get magnetized, and acquire 

 polarity, as it will be very likely to do, by standing in a verti- 

 cal position ; it is obvious, that in many cases, it might exert 

 a sensible influence upon the needle. The larger part of the 

 centre-pin which passes through the plate, should be of brass 

 or copper, and into this the small steel pivot, on which the 

 needle librates, should be firmly screwed. 



Much uncertainty and difficulty in the practice of survey- 

 ing, particularly in tracing old lines, arises from the constant 

 changing of the declination of the needle. 



This difficulty might be obviated in relation to future sur- 

 veys, by carefully observing, once or twice every year, the ex- 

 act variation of the magnetic, from the true meridian ; and 

 by establishing a true meridian line for the adjustment of 

 different instruments.* 



Such observations being made and registers of them kept, 

 at as many places as might be necessary, we should thence, 

 knowing the dates of the different surveys thereafter made, 

 have the means of determining the precise bearing of a given 

 line at any time when it might be convenient, whether the 

 survey had been made by the magnetic, or by the true meri- 

 dian. Such observations besides a practical, would have a 



* Mr. Gummere, author of an excellent treatise on the theory of surveying, I 

 think proposes something similar. See his discourse on land surveying. 



