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260 the oil-painting afterwards executed for Francis I., was the only cartoon which Leonardo had drawn since he had been in Florence.

In vain the Marchesa reminded Leonardo that he had promised to paint her portrait in oils, and begged for some little sacred subject for her studio. "You might at least," she wrote to Fra Pietro, "persuade him to paint us a little Madonna, as sweet and holy as his nature would lead him to conceive." But her entreaties met with no response. The friars waited in vain for their altar-piece, and Isabella's Madonna was never painted.

In the summer of 1502, Leonardo entered the service of Cæsar Borgia as military engineer, and travelled through Romagna—"the realm of all stupidity," as he called this province—visiting Urbino, Rimini, Cesena, and Forli, inspecting fortresses, drawing plans, and noting down any curiosities which he saw on his journey. Early in the following year he returned to Florence, and became once more absorbed in the study of mathematics. In July, he paid a visit to the camp before Pisa, and prepared elaborate plans for the construction of a canal between that city and Florence, And in January, 1504, he was present at the meeting of the artists who chose a site for Michelangelo's David. In the April following,