on the Distribution of the Magnetic Intensity. 3 



nity of buildings, or any object which I might suspect would in- 

 fluence their accuracy. I regret not having a portable dipping 

 needle to accompany the horizontal, by which means the whole 

 of the magnetic intensity at the different stations could have 

 been obtained. 



The apparatus consists of a tripod stand, a box, a thermome- 

 ter divided to REAUMUR'S scale, and two needles. In the bot- 

 tom of the box there is a circle on paper, graduated into degrees, 

 over the centre of which the needle is suspended by a silk fibre 

 of about four inches in length. The box is also furnished with 

 foot-screws for the proper adjustment of the apparatus. 



Previous to the commencement of the observations, the sus- 

 pended needle was drawn about 25 degrees out of its natural po- 

 sition, and allowed to diminish the arc of its vibration to 20 de- 

 grees, before I commenced registering the observations. The 

 needle, by which the other was drawn aside, was always placed at a 

 distance of 1 5 or 20 yards from the apparatus while the observa- 

 tions were making ; and I was always very careful to divest my- 

 self of every thing of iron during the experiments. 



With regard to Professor HANS TEEN'S method of observing, 

 described in his letter to Dr BREWSTEK, which accompanied the 

 apparatus, " to take the mean of 300 vibrations, commencing 

 with an arc of 20 degrees, and ending with an arc of 2 degrees ;" 

 this I found impossible for me at all times to do, for, as I ad- 

 vanced north on my journey, the horizontal intensity of the 

 earth's magnetism was diminishing, and the needles, particularly 

 cylinder No. I., would have almost been in a state of rest be- 

 fore I had completed 300 vibrations. I was therefore under the 

 necessity of increasing the arc as I required in going north, and 

 of decreasing it again as I came south. This circumstance 

 caused me to make experiments, to ascertain a correction to be 

 applied to the increased arc of vibration. 



As Professor HANSTEEN, in the letter before alluded to ; has 



A2 



