horizontal needle when under the influence of magnets. 353 
the hours of the day are marked in the lines of the several 
ordinates. The points determined by actual observation are 
marked with a dot, and are connected by a line, as nearly as 
I could judge, in the direction which the curve would have 
taken had more points been so fixed. When the observations 
were made, it had not occurred to me to represent them in 
this manner, or they would have been more multiplied for 
the purpose : this defect I remedied when I observed the 
deviations with the north end of the needle to the south. 
Many anomalies are here apparent, but I cannot attribute 
any of them to particular local causes, such as the moving of 
iron in or near the room. The door of the room was locked, 
so that nothing could be moved in it without my being aware 
of it ; and whenever I found any peculiarity in the deviation, 
I minutely examined the adjacent rooms to ascertain whether 
it could have been produced by any article of iron having 
been introduced or moved in them. This caution was per- 
haps unnecessary, since I had found that transferring the 
fire irons from one part of the room to another, at nearly the 
same distance from the needle, did not produce any very 
sensible effect. I have before omitted to state that there was 
no fire in the room : had there been one, the difference in 
the temperature of the iron might have produced a slight 
effect. 
That several of these irregularities were caused by the 
electric state of the clouds I have no doubt ; and for the pur- 
pose of showing how distinctly the effects are marked by a 
needle under such circumstances, I subjoin the observations 
made on the 19th, at times when, although there was no 
thunder, there were unequivocal signs of the electric state of 
the atmosphere. 
Z z 
MDCCCXXIII. 
