3 



Introduction 



A considerable portion of the MS. is taken up with "Directions for 

 using the Dipping Needle," written out at greater or less length (probably 

 according to the scientific capacity of the recipient) "for Captain 

 Pickersgill," "for Captain Bayley," "for Dalrymple" [Hydrographer to 

 the Honourable East India Company] &c. 



There is also a treatise of 26 pages "On the different forms of con- 

 struction of dipping needles." 



Besides this, there is a series of observations of the magnetic variation 

 and also of the dip, at various times, from 1773 to August 1809 (Cavendish 

 died Feb. 24, 1810). 



These observations were made for the most part only in the summer 

 months, but during that time were carried on with the greatest regularity, 

 and results for each year calculated from them. 



We also find the record of "Trials of Nairne's needle in different parts 

 of England in August, 1778." 



It was tried "in Garden, Aug. 8. In Garden of Observatory at Oxford, 

 Aug. 14. At Birmingham, in Bowling-green, Aug. 15. At Towcester, in 

 Garden, Aug. 17. At St Ives, in Garden, Aug. 18. At Ely, in Garden, 

 Aug. 18. At London, Aug. 19 and 22." From these trials he finds that 

 "Lines of equal dip should seem to run about 44 to south of west, and 

 dip should increase about 42' by going i to N.W." 



There is a long and valuable series of experiments on the magnetic 

 properties of forged iron, blistered steel, and cast iron. "Some bars were 

 got from Elwell 31! inch long, 2-1 broad, and about -5 thick. On May 29, 

 1776, one of each was made magnetical, the marked end being the south 

 pole. In trying the experiment the bars were placed perpendicularly 

 against a wall 25 inches distant from the center of the needle, 91^ to west 

 of usual magnetic north, either the top or bottom of the bar being always 

 on a level with the needle. They were kept constantly with the marked 

 end upwards till after the observations of June 30, after which they were 

 kept with the marked end downwards." 



Cavendish determines in every case the "fixed magnetism" and the 

 "moveable magnetism" of the bar, and also its magnetism when "struck 

 100 times on an anvil, falling 1-6 inches by its weight, and tried im- 

 mediately after." 



There are also 23 pages of experiments on the effect of heat on magnets, 

 and a mathematical investigation of the bending of the dipping-needle by 

 its own weight as affecting the determination of the dip, together with 

 measurements of the elasticity of steel and of glass. 



