Observations on Surveying Instruments. 275 



On one occasion, the needle of my compass was so strong- 

 ly attracted to the glass, that it was with the greatest difficul- 

 ty that I could shake it off, and when I had succeeded in de- 

 taching it, it would immediately return and adhere as firmly 

 as before. 



In this dilemma, I washed the glass on both sides, in a 

 neighboring brook, which seemed to lessen the attraction 

 considerably, but so strong was the excitement about the in- 

 strument, that near half an hour elapsed before I felt any con- 

 fidence in its accuracy. 



To obviate the errors arising from this source, by prevent- 

 ing the friction of the glass, Mr. Mullett proposes to cover 

 the face of the compass with two semicircular brass plates, 

 to turn on hinges, and so contrived that by means of a spring, 

 they can be made to fly open when required.* 



I will only add, that the surveyor's compass, or any other 

 instrument, depending on the needle, requires to be used 

 with great circumspection. 



In order to ascertain, whether the attraction of the needle in the aforemen- 

 tioned cases, was caused by the electricity excited, or by magnetism, which 

 from the strong attraction of the needle, I suspected might be a concomitant, I 

 subsequently made a variety of experiments with different substances, by a very 

 delicate electrometer, and a small needle nicely balanced. A stick of sealing 

 wax or a disk of polished glass, when excited, attracted and covered themselves 

 with iron filings, and also attracted the needle and electrometer ; but on being 

 dipped in water, they suddenly lost all their attractive power, and did not affect 

 the needle. The disk of glass, however, when excited, did not seem to have 

 its attractive power materially lessened by holding it between two brass plates 

 for several minutes, and would attract either the electrometer or the needle ; but 

 the needle seemed the most sensitive, and would be attracted by a small excite- 

 ment of the glass, when the eleetrometer, which was a small down feather, 

 would not be moved. The stick of sealing wax, or the glass when excited and 

 placed on a tea cup in a pan of water, could be drawn by a magnet from one side 

 of the pan to the other, to and fro, at pleasure. The same could also be done by 

 a piece of iron, a brass scale, or boxwood rule, or any thing else with equal fa? 

 cility. Hence it appears the attraction of the needle by the glass was not ow- 

 ing to any particular affinity of the electricity of the glass for the magnetism of 

 the needle, but that the electricity attracted it by the same virtue, and with the 

 same force that it does all other bodies. 



* This would answer, provided the glass is the only part of the instrument in 

 which the magnetic or electric influence can, in this way be developed, but I 

 have observed at times, that the whole of one part of the compass appeared to 

 be temporarily magnetized, and this is the only way that 1 could account for the 

 hasty swing and sudden stop of the needle before mentioned, when it was ap- 

 parently, not at all influenced by the glass. I have also observed in some in- 

 stances, when the needle appeared to have been disturbed in settling, that put- 

 ting the hand toward it, would attract it several degrees. I have never observ- 

 ed this curious phenomenon, except when I had been walking, and suppose it 

 may be attributed to the electricity thus excited. Mr. Mullett and others, have 

 observed the same thing in similar circumstances. 



