MAGNETIC OBSERVATIONS AND RESULTS. 225 



reading the declination directly from the needle. The needle, however, I perceived, 

 would not traverse. . Supposing the glass cover pressed upon it, I took it oiV, when the 

 needle moved freely and normally. Finding, however, the wind agitated it too much 

 to allow it to come to a state of rest, I placed the glass hack, and found, on a closer 

 examination, that it did not touch the needle. But still the needle would not traverse. 

 I again took off the glass and the needle again traversed freely. I then extended the 

 glass to its place on the needle gradually, when 1 noticed the effect of the proximity 

 was to paralyze the needle, and that in proportion to the proximity, so that when the 

 glass was in its place the motion of the needle was entirely paralyzed. The cause, 

 then, of the needle notjraversing was the influence of the glass cover in its then ab- 

 normal state. Finding the compass to be of no service for the purpose in view, I sub- 

 stituted another in its place, which I found to work well without any signs of disturb- 

 ing agency. Some days after this I had occasion to again use the first-named com- 

 pass, when I found the needle acted normally. 



The cause, then, of the disturbance above referred to was on account ot the acci- 

 dental abnormal state of the glass cover at the time, and not from any permanent dis- 

 turbing cause. It is not understood, however, what caused the abnormal condition ot 

 the glass of the first large surveyor's compass. Both it and the second surveyor's com- 

 pass were taken out of their respective boxes just before using them, and the state ot 

 the wind was by no means one of irregularity. Besides, if it had arisen from the at- 

 mosphere, what affected the one ought to have affected the other, as they had both 

 been subjected to the same handling. 



I have thought it proper to note these irregularities in the magnetic needle on ac- 

 count of its bearings upon the accuracy of surveys depending upon its normal state, 

 and the necessity of watching to see that no such disturbing causes are in operation 

 at the time bearings are taken with it. I think there can be no doubt that frequently 

 irregularities, which have been attributed to local attraction, have arisen from this 

 source, and not from the presence of metallic substances to which they have been 

 ascribed; and it is very probable, too, that these irregularities, in all such cases, have 

 been but temporary.* (See, also, Appendix K; pages 7S and 7!».) ^ 



* Since writing the foregoing I ha\ «■ Wcuuu- acquainted whh Mr. \\ . II. 1'aiue. sm\r>..r .m.l ,-ml .*u-i u. .-i . »t Slit- 



have made relative to some of the distiu b ■ ■ m used in tJw lunreyors oompass. 



"An unfavorable electrical state of the jilass cov. sin-- the n< ■<■•'.!.■ is a ver\ cuininoii can-.- <>i disriirhano . and it> 



" Whenever I 

 some disturbing i 

 rently settled, hav 







irhen the needle is disturbed, and on two occasions, in particnlar, I was unable t 



