On the Variation of the Magnetic Needle. 221 



ameter with any degree of confidence. It is however certain, that 

 its diameter has been greater than before the perihelion. On the 

 22nd of February, I observed with Clark's telescope a central part 

 quite bright, at least two minutes in diameter, and surrounded by a 

 coma nearly circular, whose entire diameter was at least six min- 

 utes. At this time, the comet was about as distant from us as it was 

 September 8th, when its apparent diameter did not exceed three 

 minutes. These facts will suffice to shew that Halley's comet is 

 far more substantial than Encke's comet, which in passing its peri- 

 helion becomes so rarefied, as to be scarcely able to reflect to us at 

 all, the light of the sun. 



Art. II. — Observations on the Variation of the Magnetic Needle, 

 made at Yale College, in 1834 and 1835; by Elias Loomis. 



About the middle of October, 1834, I commenced a series of ob- 

 servations on the diurnal variation of the magnetic needle. The in- 

 strument employed was a Variation Transit, by Dollond, belonging 

 to the College. The needle is 5.4 inches in length, and the com- 

 pass circle is graduated to quarter degrees. The azimuth circle is 

 graduated to half degrees, and has three verniers, each reading to 

 single minutes. The instrument was placed by a north window in 

 North College upon a solid block of wood, resting on the floor, and 

 so secured as to be free from all motion, except the unavoidable 

 agitation of the building. There was no fire in the apartment where 

 the instrument was placed, although its temperature was somewhat 

 affected by a fire in an adjoining room. Before commencing the 

 observations, all movable iron was removed from the vicinity of the 

 needle ; and no change was made in this respect during the contin- 

 uance of the observations. The several adjustments were carefully 

 attended to. The levels were first corrected, so that the instru- 

 ment might be turned quite round in azimuth, without sensibly mo- 

 ving the bubble in either level. I ascertained that the perpendicu- 

 lar wires of the transit, were truly perpendicular to the horizon, by 

 pointing the instrument towards a star and moving the telescope in 

 altitude. I ascertained that the horizontal wire was truly horizon- 

 tal, by causing a star to travel upon it, when the instrument was in 

 the meridian. To ascertain if the line of collimation was perpen- 

 dicular to the axis of rotation, I noted the instant of Polaris' passage 

 at the first two wires ; then reversed the axis and noted the third 

 passage. The two intervals were very nearly equal. I ascertained 



