The conquest of time and space
Book format: An electronic version of a printed book that can be read on a computer or handheld device designed specifically for this purpose.
Publisher: Date:8/5/2009 - General Books LLC
By: Henry Smith Williams
Excerpt from book: later by Gillebrand, Professor of Geometry at Graham College. Dr. Halley, the celebrated astronomer whose achievements have been recalled to succeeding generations by the periodical return of the comet that bears his namegave the matter attention, and in a paper before the Royal Society in 1692 he pointed out that the direction of the needle at London had changed in a little over a century (between 1580 and 1692) from n degrees 15 minutes East to 6 degrees West, or more than 17 degrees. Halley conclusively showed that similar variations occurred at all other places where records had been kept. He had already demonstrated, a few years earlier, that the deviations of the compass noted at sea are not due to the varying attractions of neighboring bodies of land, but to some influence having to do with the problem of terrestrial magnetism in its larger aspects. Halley advocated the doctrine, which had first been put forward by William Gilbert, that the earth itself is a gigantic magnet, and that the action of the compass is dependent upon this terrestrial source and not, as many navigators believed, on the influence of a magnetic star, or on localized deposits of lode- stone somewhere in the unknown regions of the North. Further observations of the records presently made it clear that there are also annual and even daily variations of the compass of slight degree. The fact of diurnal variations was first discovered by Mr. Graham about the year 1719. More than half a century later it was observed by an astronomer named Wales, who was accompanying Captain Cook on his famous voyageround the world (1772-74), that there is yet another fluctuation of the compass due to the influence of the ship on which it is placed. Considerable quantities of iron were of course used in the construc...