INDEX. 
Contributions to Terrestrial Magnetism. — No. VII. Containing a Magnetic Survey of a considerable 
portion of the North American Continent, by Lieut. ( since Captain ) Lefroy of the Royal Artillery. 
Page 
Introduction and Results, pages 237 to 261. 
237. Origin of the Survey; preliminary arrangements and proceedings during its course. 
241 . Additional determinations received from Major Graham, Dr. Locke, Dr. Bache and Professor Renwick. 
244. Deduction of the total Magnetic Force, in absolute measure at Toronto, as a primary base station. 
246. Deduction of the total force at Toronto in the arbitrary scale. 
247. Factors for converting the values in the arbitrary scale into absolute values, and the converse. 
248. Memorandum by Mr. Archibald Smith on a method of computing the geographical position of the 
Focus of greatest Force in North America. 
250. Results of a computation by this method. / 
251. Notices respecting the Map of the Force. 
252. Intensity of the Magnetic Force at its point of maximum in North America. 
254. The increase of the Force with the increase of Inclination shown not to be a general law. 
255. Objections to the employment of the terms “ Magnetic Pole” and “ Magnetic Equator.” 
257. Method, proposed in the Survey of the British Islands, for computing the direction of the isoclinal lines 
and the geographical distance between them, not applicable to the observations of the North American 
Survey. 
257. Description of a graphical method adopted instead. 
259. Tables showing the values of the Inclination at the intersection of meridians and parallels between 38 J 
and 66° N. Lat., and 231° and 291° East Longitude. 
260. Notices respecting Station Errors. 
Details of the observations, pages 261 to 336. 
261. Discussion of the observations of Latitude and Longitude. 
267. Discussion of the Ratios of the Force obtained by Mr. Fox’s Statical method. 
271. Discussion of the Ratios of the Force obtained by Dr. Lloyd’s Statical method. 
275. Tables of the Statical observations. 
285. General Table of the Statical results. 
287. Lieut. Lefroy s R.eport relative to the observations of the absolute horizontal Force. 
295. General Table of the absolute determinations. 
303. Horizontal Ratios obtained from the Vibrations of the magnets of the Portable Unitilar Magnetometer. 
311. General Table of the total Force in absolute measure derived from the observations of the horizontal 
component and of the Inclination. 
312. Dr. Locke’s observations of the Force referred to Toronto as a base station. 
314. General Table of the total Force at Dr. Locke’s stations. 
315. Horizontal and Total Force in absolute measure at Professor Renwick’s stations. 
317. General Table of the determinations of the Magnetic Force. 
323. Detailed statement of the observations of Inclination. 
328. General Table of the determination of the Magnetic Inclination. 
