Before Leonardo could return to the problem of flight, however, he was again faced with the necessity of supporting himself and his growing household. The small fees he received for taking on apprentices hardly covered the cost of housing and feeding them. Moreover, the equipment he had to buy for his scientific researches added further to his strained budget. So, when a servant from Francesco del Giocondo, a rich Florentine merchant, presented himself at the gate with the request that Leonardo accept a commission to paint Francesco’s wife, Leonardo was only too glad to accept. The name of Francesco’s wife was Madonna Lisa, or Mona Lisa for short. Leonardo painted her portrait on and off for the next three years. Thus, what started as a minor commission ended as the one painting—in addition to the “Last Supper”—that most people today associate with the name of Leonardo da Vinci. Having secured this work, Leonardo turned back to his studies of birds in flight and the nature of air. The soaring wings of eagles and hawks and the way they rode the currents with hardly a dip of their spread wings guided Leonardo’s thinking from pure mechanics to machines that act more on the principle of the glider. He proposed to write a treatise on the nature of birds’ flight, and, with his usual thoroughness, he began to weigh, dissect, and reconstruct various types of birds and their wing structure. He realized that one of the main difficulties of gliding was maintaining balance, or, more accurately, maintaining the center of gravity. From previous observations Leonardo had noted that man is capable of making the same motions that a bird does. He had also measured the strength of a man’s legs and had calculated that man has twice the power in his leg muscles that he needs for standing. Consequently he began to redesign his machine making use of man’s arms and legs to operate or “flap” the wings instead of standing him on a platform. The first of Leonardo’s new designs was a sort of harness apparatus strapped across the shoulders of the flyer who was supposed to be able to keep himself balanced by moving the lower part of his body. He could manipulate the flight by handles that were connected to the flexible, outer parts of the wings. These wings were designed from the webbed wings of the bat. Surprisingly enough, this device closely resembled the experimental gliders used by Otto Lilienthal almost four centuries later in Germany.