Search Loci: Convergence:
If a nonnegative quantity was so small that it is smaller than any given one, then it certainly could not be anything but zero. To those who ask what the infinitely small quantity in mathematics is, we answer that it is actually zero. Hence there are not so many mysteries hidden in this concept as they are usually believed to be. These supposed mysteries have rendered the calculus of the infinitely small quite suspect to many people. Those doubts that remain we shall thoroughly remove in the following pages, where we shall explain this calculus.
Kepler: The Volume of a Wine Barrel
Solving the Problem of Maxima: Wine Barrel Design
To optimize the volume of the wine barrel, Kepler simplified the problem. He approximated the barrel by a cylinder with the diagonal measurement \(d=SD\) as above, \(r\) the radius of the base, and \(h\) the height of the cylinder. In modern notation, the formula for the volume \(V\) of a cylinder is:
Using the Pythagorean Theorem, we can write an equation relating \(d,\) \(h\) and \(r\):
Now solve for \(r^2\):
Hence, we can deduce the formula for the volume \(V\) as a function of \(h\) and \(d\):
For fixed \(d,\) the following animation shows how the volume \(V\) changes when we modify the height \(h\) of the barrel.
Figure 7. For a fixed value of \(d,\) the volume \(V\) is a function of height \(h\). The graph at left shows how, for a fixed value of \(d,\) changes in height \(h\) result in changes in volume \(V\). The figure at right shows how, for a fixed value of \(d,\) changing the height \(h\) changes the radius \(r\) and the shape of the cylindrical barrel. (For instructions in English, please see Kepler: Doliometry, Volume of a Wine Barrel at MatematicasVisuales!)
Next, Kepler asked: "If \(d\) is fixed, what value of \(h\) gives the largest volume \(V\) ?" (Toeplitz, p. 83). After making some calculations and comparing them, he decided the answer was:
According to Toeplitz (p. 83):
That the volume function changes very slowly near its maximum value is illustrated in the graph below.
Figure 8. For a fixed value of \(d\), the graph of volume \(V\) as a function of height \(h\) illustrates that, near the maximum volume, small changes in height \(h\) result in small changes in volume \(V\).
Kepler tabulated values obtained by calculation to reinforce his idea that the volumes of such cylinders with equal diagonals change very little in the neighbourhood of a maximum (Baron, p. 116).
Figure 9. Kepler's table of volumes of wine barrels of various heights (altitudes) from p. 66 of his 1615 Nova stereometria. View this page and the whole book at The Posner Memorial Collection: Kepler's Nova Stereometria. (Image used by permission of the Carnegie Mellon University Libraries)
But what about the barrel of wine Kepler had purchased for his wedding in Austria? Was it priced fairly? According to Toeplitz (p. 83):
This, then, was the major contribution by Kepler: He noted that as the maximum volume was approached, the change in the volume for a given change in the dimensions became smaller. Some years later, Fermat would consider maximum and minimum problems of this kind from a similar point of view.
Cardil, Roberto, "Kepler: The Volume of a Wine Barrel," Loci (June 2010), DOI: 10.4169/loci003499