The end of the week found them a hundred and fifteen miles closer to the Pole. They had encountered two leads, but had managed to get across without great difficulty. One of the sledges had been badly damaged, and it was resolved to break it up, and use the parts in repairing the other turnouts. Two of the dogs were sick, and had to be killed. The next day the weather changed, and for forty-eight hours they struggled on through a heavy snowstorm, with the wind fortunately on their backs. During this storm one of the sledges fell into some open water, and three dogs were drowned, while a small portion of the outfit went out of sight into the Arctic Sea. "All hands must be more careful after this," said Barwell Dawson. "As we advance, going will probably become more treacherous. Keep your eyes wide open." As soon as it cleared off, Professor Jeffer brought out his sextant and his artificial horizon (a pan of mercury), and took an observation. He announced that they were close to the eighty-fourth degree of north latitude. "That means we have but six more degrees to cover,--about four hundred miles," said Chet. "Professor, will you explain how you take the observation?" asked Andy. "To be sure, certainly," was the reply of the scientist. "It is very easy when one knows how. Here is the sextant, shaped, as you can see, like a piece of pie. The curved side has a scale on it, which is just one-sixth of a circle, hence the name of the instrument. Here is a telescope which is adjustable, and here are two glasses, one for the rays of the sun, or a star, and one for the horizon. At sea, I would use the natural horizon, but that is impossible here amongst the ice and snow, and so I use an artificial horizon made of a pan of mercury. "When I want to take an observation, I watch my chronometer and wait until it is exactly twelve o'clock. Then I point the sextant in such a fashion that the rays of the sun, reflected downward, seem to meet or 'kiss' the horizon. As soon as I have the light of the sun in a direct range with the horizon, I use this thumbscrew, which sets the scale below, which, as you see, is divided into degrees, minutes, and seconds. As soon as I have read the scale by means of this magnifying glass, I consult this book I carry, the Ephemeris, or Nautical Almanac, and knowing the altitude of the sun, I readily calculate just where we are located, in degrees, minutes, and seconds north latitude." "It's certainly a great instrument," said Andy. "I'd like to try it some day." "You shall do so," answered Professor Jeffer, and the very next day he allowed Andy to aid him in getting a true sight, and showed the boy how to work out the necessary calculations, and also make some allowances,--for such observations are not absolutely perfect in themselves.