Hudson's Strait and Bay. 



267 



netic compass. At the western end of the Strait it becomes so sluggish 

 as to be almost useless. The Sir Wm. Thomson card was used by 

 Captain Gordon, and was found to work admirably when properly 

 compensated. 



The reason of the difficulty with the compass is, that from* the 

 proximity to the magnetic pole, the horizonal directive force of the 

 earth's magnetism, which alone directly affects the compass needle, is 

 very small compared with the whole magnetic force ; consequently, the 

 effects of induced magnetism in the iron of the ship on the compass 

 needle becomes very large in comparison with the direct action above 

 mentioned ; the result being, that in an imperfectly compensated com- 

 pass, the error due to local attraction is very greatly increased. 



The means of correcting this error in the Sir Wm. Thomson bin- 

 nacle are perfect and easily mastered, and the system is such that the 

 compass can, after the first voyage or two, be perfectly compensated by 

 using certain proportions of soft iron bars and magnets, as correctors, . 

 the proportion having to be determined by actual observation and 

 experiment on the voyage. Of course, in that region no opportunity 

 should be lost of taking azimuth observations, both stellar and solar. 



Our last stopping-place, previous to crossing Hudson's Bay, was at 

 the Digges Islands at the extreme western end of Hudson's Strait. 

 Here we remained several days for the purpose of taking on balk t, 

 testing the chronometers and making a survey of the islands, which 

 had never before been surveyed. 



The general appearance of the landscape of these islands is similar 

 to most of the country around Hudson's Strait, presenting bold, rocky 

 shores utterly devoid of vegetation, except moss, lichens, stunted willow- 

 bushes, small plants and grass in the lower valleys. This whole region 

 is desolate-looking in the extreme. I was often amazed to see the 

 amount of animal life in existence there, and wondered how it all 

 managed to find sustenance. 



The rocks of Digges Islands are mainly Laurentian gneiss. The 

 bare hills of which they are composed are divided into several detached 

 groups by straight, transverse valleys, cutting well down toward the 

 sea-level, thus giving the appearance of separate islands, when viewed 

 from a distance. The greatest length of the islands lies about east and 

 west (true). As this is also the commonest direction of the strike of 

 the gneiss, most of which is red, and also of the glacial stria?, the 

 islands have become divided by longitudinal valleys, some of which, too, 

 were traced in nearly straight courses for several miles. 



It was learned by a survey which we made that there are two of these 



