From previous tests each of the boys knew the exact stretch of ground covered in one of his strides[#] and Dick's stride being sixty inches, even though he was a six-footer, and five feet being a most convenient multiple, it was to be his duty to keep account of the distances between observation points or stations. For this purpose he carried an instrument used in checking off the number of coal bags hoisted on board during coaling ship, and with each step taken with his left foot he recorded it by pressing on the lever with his thumb. The tally was so small it could be carried unobserved in the palm of the hand. Besides the tally Dick carried a small pocket note-book, conveniently ruled, in which he entered his data and from which, on their return, they would be able to make a very comprehensive sketch of their travels.
[#] A pace is the distance between footsteps; a stride the distance between the spot where one foot strikes the ground and the next succeeding fall of the same foot; a stride is therefore the equivalent of two paces.
Henry was provided with a small prismatic compass by means of which he read the angles from each selected point to the next station. With these simple instruments they could accomplish their work and arouse no suspicion, at least in the minds of any ordinary native with whom they were liable to come in contact.
There was but one trail for them to follow from their point of departure, and it led to the town of Copey. To follow this trail the first day and plot in the cross trails between it and the Massacre River on the following days was their intention, and as the sun rose in a soft pink cloud of color, with shotguns under their arms, game bags over their shoulders, and the heavy Colt's forty-fives strapped to their right thighs, the young surveyors started out on their quest with an eagerness born of youth and enthusiasm.
CHAPTER XVIII
MEXICAN PETE AGAIN
The method followed by the two marines was very simple. Having selected a landmark some distance ahead of them on the trail, Henry, with the sight-leaves of his compass raised, would look through them towards the point and read the azimuth or angular direction with respect to the north and south line, or meridian. This angle was called the bearing of the point or station.
Starting at the Captain of the Port's house they named their point of departure "A," and sighted upon a distant tree, calling it station "B." A line drawn from "A" to "B" would form an angle with another line passing through station "A" and the north pole. This angle was read off in degrees on the compass-card from north going around in the same direction as the hands of a clock, and there would be two methods of recording it. They could state the whole angle as read from the compass, which would then be the true azimuth of station "B," or they could note the true bearing of the line A-B. The true bearing of a line is that angle less than ninety degrees which the line makes with the true meridian. The boys decided to use the true azimuth in their data.
Dick, having made the entry in his book, started marching towards "B," pressing on his tally register with every fall of his left foot. Reaching "B," the number of his strides were entered, a new sight taken, and the march resumed.
Where trails crossed or joined the route, their bearing was jotted down. Features of the country to one hundred yards either side of the trail were kept under observation; houses, corrals, streams, bridges and their nature, cultivated fields, swampland, all were noted carefully.