The ingenuity of Franklin was also exerted very successfully in the rectification of smoky chimneys. In his essay on the causes and cure of such chimneys, written on his last ocean voyage, he resolved the causes into no less than nine heads, and stated with his accustomed perspicuity and precision the remedy for each cause. In his time, the art of properly carrying off smoke through chimneys was but imperfectly understood by ordinary builders and mechanics, and it was of too humble a nature to tempt discussion by such men of science as were capable of clearly expounding the physical principles upon which it rested. It was not strange, therefore, that Franklin, who deemed nothing, that was useful, to be beneath the dignity of philosophy, should have acquired in his time the reputation of being a kind of "universal smoke doctor" and should have been occasionally consulted by friends of his, such as Lord Kames, about refractory chimneys. The only smoky chimney, that seems to have completely baffled his investigation, recalls in a way the philosopher, who thought that he had discovered a new planet, but afterwards found that what he saw was only a fly in the end of his telescope. After exhausting every scientific resource in an effort to ascertain why the chimney in the country-house of one of his English friends smoked, Franklin was obliged to own the impotence for once of his skill; but, subsequently, his friend, who made no pretensions to the character of a fumist, climbed to the top of the funnel of his chimney by a ladder, and, on peering down into it, found that it had been filled by nesting birds with twigs and straw, cemented with clay, and lined with feathers. Nor was the attention given by Franklin to ventilation by any means confined to chimneys. Air vitiated by human respiration also came in for a share of it. Describing an experiment by which he demonstrated the manner in which air affected in this way is purified, Alexander Small said: The Doctor confirmed this by the following experiment. He breathed gently through a tube into a deep glass mug, so as to impregnate all the air in the mug with this quality. He then put a lighted bougie into the mug; and upon touching the air therein the flame was instantly extinguished; by frequently repeating the operation, the bougie gradually preserved its light longer in the mug, so as in a short time to retain it to the bottom of it; the air having totally lost the bad quality it had contracted from the breath blown into it.