Page:Popular Science Monthly Volume 27.djvu/244

230 The depth to which water will freeze has never yet been determined; but measurements of the formation of ice which are to be carefully made at the observing-stations will, no doubt, materially assist in a determination of this important question.

At Churchill the harbor-ice forms, on an average, about the middle of November, and breaks up about the middle of June, and these two dates may therefore be taken as marking the extreme limits of the season during which that harbor may be used.

With regard to the time consumed in making the passage through the strait, it is necessary to note that, had the Neptune gone direct from Cape Chudleigh to Churchill, instead of coasting and working across the strait, there would have been no greater delay on account of the ice than forty-eight hours at the most; but, at the same time, no ordinary iron steamship, built as the modern freight-carrier is, could have got through the heavier ice without incurring serious risk, if not actual disaster.

There is one matter to which Lieutenant Gordon draws attention that will require the serious consideration of mariners navigating these waters, namely, that in working through the strait, especially at the western end, he found the ordinary compass so sluggish as to be practically useless. The Sir William Thomson card, however, worked admirably when properly compensated. The reason of this difficulty with the ordinary compass is that, from the proximity to the magnetic pole, the horizontal directive force of the earth's magnetism, which alone directly affects the compass-needle, is very small compared with the whole magnetic force, and consequently the effect of induced magnetism in the iron of the ship on the compass becomes very large in comparison with the direct action above mentioned, the result being that, in an imperfectly compensated compass, the error due to local attraction is very greatly increased. The means of correcting this error in the Sir William Thomson binnacle 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. All steamships making the voyage through the strait, Lieutenant Gordon therefore concludes, should have one of these compasses as a standard, and the captains should familiarize themselves with the methods of correcting them, and, as opportunity offers, take azimuth observations, both stellar and solar.

Great caution will have to be observed by all vessels approaching the strait in thick weather, owing to the strong southward current there prevailing, which, during the forty-eight hours the Neptune was lying-to, swept her forty miles out of her course by dead-reckoning, showing that the amount of southerly set exceeds that indicated by the Admiralty directions. Then, again, the tides rise to a