222 On the Variation of the Magnetic Needle. 



that the line of collimation was in the same vertical plane with the 

 meridian, as marked on the compass. This was done by pointing 

 the telescope downwards towards the divisions on the compass, the 

 focal distance being regulated by a small lens fitted to the object 

 glass, and the central wire of the transit was made to coincide with 

 the two zeros on the compass. Having made these preparatory ad- 

 justments, the instrument was finally placed upon the meridian, by 

 noting the passage of Polaris. A meridian mark was fixed upon, at 

 a considerable distance, and for verification, the passage of Polaris 

 was repeatedly observed. As it required considerable time to turn 

 the instrument in azimuth so as to read off by means of the verniers, 

 these were seldom employed, but instead of them I used a com- 

 pound microscope, and estimated the fraction of a quarter degree, 

 (the smallest division on the compass,) by my eye. After some 

 practice, I was able to do this with considerable accuracy, so that, 

 as I judged, I was not liable to an error of more than one minute. 

 As the first observations were necessarily imperfect, those which 

 were made during the month of October, 1834, were rejected. A 

 Fahrenheit's thermometer was placed about two feet above the 



i 



transit, within the building, and was always observed at the same 

 time with the needle. These observations were intended to be 

 made at every hour of the day, from five or seven o'clock in the 

 morning, till ten at night ; yet some failures were absolutely una- 

 voidable. At some hours of the day, the observations were contin- 

 ed uninterruptedly for a month together ; while at a few hours, the 

 observations were made only about half the time. This fact will 

 explain some apparent anomalies in the following tables, particular- 

 ly in the observations with the thermometer. During the period em- 

 braced in these observations, the needle has repeatedly suffered a 

 sudden and irregular deflection to the amount, in three instances, of 

 more than a degree. A particular account of these irregularities will 

 be given in the latter part of this article : and they have been ex- 

 cluded from the means in the following table, the object there being 

 to exhibit the regular diurnal variation. To determine the influ- 

 ence of the building, I took the instrument out into the President's 

 garden, where it was supposed the local attraction must be small, 

 if any, and made repeated observations. The influence of the 

 building was determined to be 1° 21' 41", which has accordingly 

 been added to all the observations. The instrument was also carri- 

 ed out of the city, to a situation remote from any building, and near- 

 ly the same result obtained. 



