40 FJeclro-Magnelic 'Expenmenls. 



be deficient, or his redundnnt ; and I think, and the reader wiPl 

 most assuredly think alst), that Mr. Holdred, after the hold asser- 

 tions which he lias made, is imperiously called upon to demon- 

 strate which olthe two is the case. Though Mr. Holdred's work 

 has been the result of forty years experience and consideration, 

 he has not applied his rules to those cases in the extraction of 

 roots where there is any real difficulty. 



Such as wish to see the rules for extracting the roots of equa- 

 tions, derived by me from Sir Isaac Newton's method of trans- 

 forming equations, may consult my Analytical and Arithmetical 

 Essays, where the transformation of equations, and the determi- 

 nations of the limits of their roots, I hope are fully considered. 



VI. On the Electro- Magnetic Experiments of MM. Oersted 

 and Ampere. By Mr. Hatchetf. 



X HE use of the compass in France takes date from the year 1260. 

 The principal part of this instrument, as the reader is aware, con- 

 sists in a magnetized steel needle, of the form of a very elongated 

 lozenge. This needle, moveable round a vertical axis, brings 

 itself on every spot of the earth to an equilibrium in a vertical 

 plane, which is named the magnetic maridian. The angle which 

 this plane makes, with that of the astronomic meridian of the 

 place where the observation is made, is called the declination of 

 the compass. In 1580, this declination was at Paris 11° 30' to- 

 wards the west ; in 1663, nothing; and in 1819 (22d April), 

 22" 19' west. If the declination of a magnetised needle changed 

 neither with time nor place, or at least if the changes were made 

 according to known laws, the science of navigation would possess 

 an instrument of simple construction, easy to observe with, and 

 precious indeed to mariners, who would find in a needle, the in- 

 trinsic value of which is almost nothing, the only means of steer- 

 ing their course when night and clouds veil the sky. Philosophers 

 at first endeavoured, but in vain, to discover the cause of the 

 phaenomena which the magnetised needle presents. They, never- 

 theless, succeeded in giving to a bar of steel that singular pro- 

 perty of the natural magnet, of taking at each place of the earth 

 a position, the diurnal or secular variations of which are perio- 

 dical. They have studied and measured with care the magnetic 

 attractions and repulsions. The labours of Coulomb, the instru- 

 meiUs invented by that celebrated philosopher, and those which 

 M. Lenoir, the distinguished artist of the Bureau of Longitude, 

 has executed, have considerably improved the science of mag- 

 netism. M. CErsted, Professor of the University of Copenhagen, 

 has just opened a new field to the inquiries of philosophers. It is 



to 



