On level, undulating and moderately broken ground, the tops of the trees, though they may be one hundred and fifty feet from the ground, are level as the top of a hedge. Even an isolated hill if it be rounded in shape presents hardly better facilities, the trees at the base and on the sides, in their effort to reach the sunlight grow taller than those on the summit, and there is no one tree that commands all the others.
If however an isolated hill of several hundred feet in height be found, its steep sides culminating in a sharp peak, one day's work by three or four good axmen, in cutting neighboring trees, will prepare the way for a study of the general relief and topography of the adjacent country. If after these preliminaries have been completed the engineer imagines that he has only to climb the tree and sketch what he sees, to obtain reliable knowledge of the country, he is doomed to serious surprises in the future. If he makes the ascent during the middle of the day, he will, after he has cooled off and rested from his exhausting efforts, see spread out before him a shimmering landscape in which the uniform green carpet and the vertical sun combined, have obliterated all outlines except the more prominent irregularities of the terrene, and have blended different mountain ranges, one of which may be several miles beyond the other, into one, of which only the sky profile is distinct. Naturally under these conditions estimates of distance may be half or double the truth.
There are two ways of extracting reliable information from these tree-top reconnaissances. If it be in the rainy season the observer must be prepared to make a day of it, and when he ascends the tree in the morning he takes with him a long light line with which to pull up his coffee and lunch.
Then aided by the successive showers which sweep across the landscape, leaving fragments of mists in the ravines, and hanging grey screens between the different ranges and mountains, bringing out the relief first of this and then of that section, an accurate sketch may gradually be made. The time of passage of a shower from one peak to another, or to the observer, may also be utilized as a by no means to be despised check upon distance estimates.
If it be the dry season, the observer may take his choice between remaining on his perch in the tree from before sunrise to after sunset, or making two ascents, one early in the morning and the other late in the afternoon. In this case the slowly dispersing clouds of morning, and the gradually gathering mists at sunset, together with the reversed lights and shadows at dawn and sunset, bring out very clearly the relief of the terrene, the overlapping of distant ranges, and the course of the larger streams.
This kind of work cannot be delegated to anyone, and besides the arduous labor involved in climbing the huge trees, there are other serious annoyances connected with it. The climber is almost certain to disturb some venomous insect which revenges itself by a savage sting which has to be endured; or he may rend clothes and skin also, on some thorny vine, or another, crushed by his efforts, may exude a juice which will leave him tattooed for days; then, though there may not be a mosquito or fly at the base of the tree, the top will be infested with myriads of minute black flies, which cover hands and face, and with extremely annoying results. On the other hand the explorer may as a compensation have his nostrils filled with the perfume of some brilliant orchid on a neighboring branch; and there is a breezy enjoyment in watching the showers as they rush across the green carpet, and in listening to the roar with which the big drops beat upon the tree tops.
The special phase of field work which fell to my personal lot was entirely reconnaissance, consisting of canoe examinations of all streams in the vicinity of the line of the canal, to determine their sources, character of valley, and approximate water shed; of rapid air-line compass and aneroid trails, to connect one stream, or valley head with another, or furnish a base line for a general sketch plan of a valley; and of studies of the larger features of the terrene, from elevated tree tops.
The last has been already described; in the second the experience was very similar to that of the parties in running main lines. On these occasions three or at most four hardy huléros (rubber hunters) comprised the party, two carrying the blankets, mosquito bars and provisions for several days, and one or two cutting the lightest possible practicable trail and marking prominent trees.
In a day's march of from five to eight miles, and this was the utmost that even such a light, active and experienced party could cover in one day, every possible and some almost impossible kinds of traveling was encountered, and thoroughly exhausted men crept into their bars every night.
The canoe reconnaissances were more agreeable, though some most unpleasant as well as most enjoyable memories are connected with them.