During their three years' stay his men will be on the watch for opportunities to advance northward, and if they find none, they intend to make such a study of currents, ice, and seasons as will give the cue to others in after years. The principal difficulties in pushing far northward may be summed up in a few words. The attempt must be made in summer (the Arctic day), when the ice is liable to break up. A boat must therefore be carried, and this makes the sledge train heavy. The ice to be crossed is extremely rough, and explorers have not been able to find smoother spots of any considerable size. By rough we mean that it is covered with deep rifts, blocks and snow drifts from five to twenty feet or more in height, and these impediments cover the surface so closely as to leave no alternative but a slow tugging of the sledges over the most available parts of them. The English expedition found these drifts to lie directly across their course, having been formed by a west wind. The labor of crossing them is performed with the thermometer far below the freezing point. There is no fire, provisions have to be carefully husbanded, sleep is dangerous unless frequently broken, and if one of the party breaks down, the strength of the whole is seriously diminished, while its task is greatly increased. Such has been the history of exploration up to within 400 miles of the pole, and it is at least probable that many of these difficulties will be intensified as that point is reached. The north pole may now be considered to occupy the centre of an area 800 miles in diameter, the condition of things within which it is not possible even to conjecture. We may plausibly suppose (1) that it is not land, for the ice of the Arctic sea is never more than 150 feet thick, and there are no glaciers; (2) that it is a shallow sea; and (3) that the precipitation of moisture in the centre must be considerable, as the ice is moving in all directions from the centre during the summer. The theory of an open sea at the pole is now discarded by most scientific men, and, we believe, by all experienced explorers except Hayes. In the present state of knowledge it rests upon the presumption that the polar sea is very shallow, so that the deep and warm currents which are known to enter the Arctic ocean may be forced to the surface there; and that the ice drift removes the ice as fast as it forms. EXPLORATION NOTES. THE Portuguese government has decided to spend $100,000 on a scientific expedition to Central Africa.