360 



PHILOSOPHICAL TRANSACTIONS. 



[anno i77a, 



fruitlessly, among other methods, the drinking of tar-water and sea-water, of 

 each of which, he says, he did not drink, less than a hogshead." 



XXXV. Experiments on two Dipping Needles, made after a Plan of the Rev. 

 Mr. Mitchel, F. R. S., Rector of Tkornhill, and executed for the Board of 

 Longitude by Mr. Edw. Nairne, of Cornhill, London, p. 476. 



The magnetic needles were 12 inches long, and their axes, the ends of which 

 were of gold allayed with copper, rested on friction wheels of 4 inches diameter, 

 each end on 1 friction wheels, which wheels were balanced with great care. The 

 ends of the axes of the friction wheels were likewise of gold allayed with copper, 

 and moved in small holes made in bell-metal; and opposite the ends of the axes 

 of the needles, and the friction wheels, were flat agates, finely polished. Each 

 magnetic needle vibrated in a circle of bell-metal, divided into degrees and half 

 degrees, and a line passing through the middle of the needle to the ends pointed 

 to the divisions. The minutes set down in the experiments were, by estima- 

 tion, as the third of half a degree is counted 10 minutes. The instruments 

 were carefully placed, so that the needles vibrated exactly in the magnetic meri- 

 dian. The 2 needles were nearly balanced before they were made magnetical; 

 but, by a curious contrivance of Mr. Mitchell, of a cross fixed on the axes of 

 the needles (on the arms of which were cut very fine screws, to receive small 

 buttons, that might be screwed nearer to or farther from the axis) the needles 

 could be adjusted both ways, to a great nicety, after they were made magnetical, 

 by reversing the poles, and changing the sides of the needle. 



First set of experiments made April 21, 1772, by 

 Edw. Nairne, at his house, N° 20, Cornhill. 



/ "Second set of experiments, with that side of the in- 

 strument to the east, which was to the west in the first 

 observation. 



Here the ends of the axis touched the agates 



Third set of experiments, in which the poles of the 

 needle were reversed, but the same side of the instru- 

 ment to the east, as in the second set of experiments, 

 and the needle rather more magnetical, being touched 

 with a larger set of magnets. 



