274 Observations on Surveying Instruments. 



The common method of carrying the compass for conven- 

 ience and for the protection of the glass, particularly in the 

 woods, is to throw it over the left arm, with its face towards 

 the body, holding one sight-vane in the hand, while the other 

 lies across the arm above the elbow. 



In this position, one part of the glass will frequently come 

 in contact with the covering of the body ; and I have found 

 by abundant experience, that it is the part thus excited, which, 

 in a dry atmosphere, very often produces the aberrations of 

 the needle so much complained of, the cause of which has 

 generally been considered so inexplicable, 



I believe, that in nine cases out of ten, where local attrac- 

 tion is suspected, the surveyor need not look beyond his in- 

 strument for the cause. 



Sometimes from this cause, when the compass is set, and 

 the needle let down on to the centre-pin, it will swing hastily 

 around to a certain position, where it will suddenly stop, and 

 remain for several minutes, until the excitement appears to 

 have in some measure abated, when it will leisurely move off 

 and apparently assume its proper position. At other times, 

 when the surrounding atmosphere is drier, and circumstan- 

 ces seem more favorable to the developement of the disturb- 

 ing cause, the needle when lowered on to the pivot, will fly 

 immediately to some point in the glass, and attach itself so 

 closely by one of its ends, that it is with difficulty, that it can 

 be immediately removed. 



Note. — With a view to ascertain whether the attracting cause is developed 

 by friction in all compass glasses alike, I lately made several experiments in 

 company with Mr. Mullett ; the result of which was, that out of the four differ- 

 ent compasses which we examined, three, by rubbing a few seconds on any 

 part of the glass with silk or woollen cloth, would attract the needle from twen- 

 ty to eighty degrees, and hold it in contact with the glass from five to fifteen 

 minutes. The other glass, by rubbing, showed no signs of attraction, either for 

 the needle or electrometer — neither would an excited stick of sealing wax, nor 

 a disk of polished glass, when excited, affect the needle through it ; although 

 they would, through the other glasses at a greater distance. The glass was 

 then placed over the other needles with the same result. Hence we inferred 

 that this glass had no affinity for the electric or electro-magnetic fluid, and that 

 it was impervious to it. 



Neither of us having ever before used a compass glass that could not be ex- 

 cited, this singular exception of the fourth glass induced us to try another ex- 

 periment, to see if a steel magnet of the same attractive power as the sealing 

 wax, in relation to the other needles, when held at a certain distance, would at- 

 tract the needle through this glass. The result proved that it would vary sensi- 

 bly. Hence then, if the cause of the attraction of the needle, developed in the 

 glass and sealing wax, be magnetism, there must be two kinds ; one that can 

 •communicate its influence through this glass, and another that cannot. 



