"These porters are known as 'pagazi,' and are a necessary adjunct of every expedition in the interior of Africa. Sometimes it is impossible to procure a sufficient number, and the traveller may be delayed weeks or months while waiting for them. On the road they must be watched very carefully, to see that they do not desert with their burdens; and, in order to prevent this, the rear of a caravan must be brought up by a trusty guard. A great part of the troubles of all African explorers is due to the pagazi, and more than one expedition has been completely broken up by their misconduct.
"Sometimes they desert in a body, and the traveller who has gone to sleep, with a hundred or more porters in his employ, has risen in the morning to find his camp deserted and not a man to be found. In this dilemma he must wait till new porters can be hired, or he may be obliged to destroy a large part of his goods."
"Wouldn't it be possible for him to sell them to some of the native chiefs in such an emergency, instead of destroying them?" one of the boys inquired.
"Perhaps he could do so," the Doctor answered; "but he would obtain a very small price for them, as the chiefs would know he was in a great strait and must be rid of them. Such a practice would encourage desertions, as the local chiefs would be in collusion with the porters, and no traveller could get through in safety. It is an invariable rule with the Portuguese and Arab traders in Central Africa to destroy all goods that they are unable to carry by reason of the desertion of their pagazi. It is their only way of insuring themselves against certain loss in future journeys, and they are very particular in observing it."
Frank asked if they were to have any scientific instruments, such as were usually carried by explorers in strange countries. Doctor Bronson replied that they would certainly do so, but he had not yet made out his list of what would be wanted.
"For the first part of our journey," said the Doctor, "we shall be in a region that has been explored sufficiently, so that its principal geographical positions are known, and there will be very little occasion for instruments. But later on our route will be much like a voyage on the ocean, and we must find out 'by observation,' as the navigators say, where we are. For this purpose we can imagine that we are going on a ship, and must have the instruments that a ship usually carries."
"I understand," said Fred. "We will have a quadrant or a sextant for ascertaining the position of the sun, just as a captain does at sea. But will the irregular line of the land serve us for a horizon, as the line between sea and sky serves the mariner?"
"Certainly not," answered the Doctor, with a smile; "and to meet this difficulty we employ the artificial horizon."
"How is it made?" one of the youths inquired.
"It is a very simple affair," the Doctor answered; "it is nothing but a horizontal mirror, and is constructed in two or three ways. It may be an ordinary mirror or looking-glass, in a frame adjusted upon screws and set round with spirit-levels, so that it can be brought to the proper position, or it may be a basin of mercury. A tub of water may be made to answer in an emergency, but it is not easy to get a reflection from it of sufficient distinctness for purposes of observation. With the artificial horizon and a sextant the altitude of the sun or of a star may be readily obtained. Half the angular distance between a star and its image in the artificial horizon is equal to the altitude of a star above the real horizon."