The diagram shows the location of the dam and contour lines of the proposed reservoir, the zero and datum line being taken as the height of the water in the creek at the site of the dam and the contours show the water line of the Reservoir for each two feet in height of the dam above the datum, the number of the contour being its vertical elevation in feet above the zero plane.

We will suppose the diagram to be drawn to a scale of say 20 ft. to one inch and that the Reservoir is required to hold a given number of thousand gallons.

It is evident that the first step in the solution of the problem will be to determine to what height or contour the water must be raised in order that the Reservoir shall have a capacity of the desired number of thousand gallons. It is at once seen that this operation is exactly the same as the already explained for finding the volume of excavation from original contours since, by passing the horizontal parallel planes whose intersection with the surface of the ground form the contour lines, we have divided up the capacity of the Reservoir into a number of continuous vertical prismoids whose bases are the areas included within the respective contours and whose common length is the vertical distance between the equidistant cutting planes, in this case being one foot.

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