Caches and Depots.
Reconnoitring Barren Countries by help of Porters and Caches, -The distance to which an explorer can attain in barren countries depends on the number of days' provisions that he can carry with him. Half of his load supports him on his way out, the other half on his way home. But if he start in company with a laden porter, he may reserve his own store and supply both himself and the porter from the pack carried by the latter. When half of this is consumed, the other half may be divided into two equal portions. The one is retained by the porter who makes his way back to camp, consuming it as he goes, and the other is cached (see " Caches ") for the sustenance of the traveller on his return journey. This being arranged, the traveller can start from the cache with his own load of provisions untouched, just as he would have started from the camp if he had had no porter to assist him. It is evident a process of this description might be frequently repeated ; that a large party of porters might start, and by a system of successive subdivisions, they could enable the traveller to reach a position many clays' journey distant from his camp, with his own load of provisions and with other food placed in a succession of caches, for the supply of his wants all the way home again. The principle by which this may be effected without waste, is to send back at each successive step the smallest detachment competent to travel alone, and to do this as soon as one half f of their load of food has been consumed by the whole party. Then, the other half is to be divided into two portions ; one consisting of rations to supply the detachment back to the previous cache, whence their journey home has been provided for, the other portion to be buried, to supply rations for the remainder of the party, when they shall have returned (either all together or else in separate and successive detachments) back to the previous cache, whence their journey home has also been provided for. An inspection of the Table which I annex (p. 307) makes details unnecessary. The dotted lines show how the porters who first return may be dispatched afresh as relief parties. I give, in the table, a schedule of the three most important cases. In these the regular supply of two meals per diem, and a morning and an afternoon journey, are supposed. I wrote a paper on this subject, which is published in the `Royal Geographical Society's