Although cold may not be so frequent a cause of asthma as was formerly supposed, low temperature undoubtedly acts as an irritant upon the terminal branches of the respiratory nerves, especially the pneumogastric, and in the manner just described may produce spasmodic contraction of the bronchi. The effect of cold is of course much more deleterious when it is associated with sudden changes and diminished barometric pressure, high winds from the east and north being particularly prejudicial. Aside from its meteorological characteristics, the locality itself exercises a potent influence in the production of asthma; and here, again, we have an example of the capricious character of the disease. A patient who for years has suffered with asthma may change his residence and find immediate relief, but of the special factors which engender the disease in one place and cure it in another we know as yet but little. It is, however, a generally acknowledged fact that removal from the country to a crowded city will often diminish the severity and frequency of the attacks, and English writers mention numbers of cases of asthma which have been permanently cured by a prolonged residence in the foggy atmosphere of London. A very slight change is often sufficient to afford relief, and sometimes removal to another part of the same city is all that is necessary. The town of Aiken in South Carolina is divided by a ravine into two sections: the elevation, soil, and exposure are alike in almost every respect, but persons have been known to suffer severely with asthma on one side and to enjoy perfect exemption from it on the other. A gentleman who resides at Bath in the same neighborhood is perfectly free from asthma at his home, but invariably has an attack as soon as the train begins to cross the Savannah River at Augusta, which is only a few miles distant. More remarkable still is the case mentioned by a French writer of a young man who was unable to sleep in the front rooms of a house without having a paroxysm, but who did not experience the slightest inconvenience when he occupied the back rooms. Although removal to the city frequently affords relief, there are exceptions to the rule, and many cases are recorded where a change of residence to the country has effected a cure. Ozone, of which but little is as yet known, is supposed by some to be a cause of asthma, and it is not unlikely that the relief afforded by removal to a large city may be partly due to the relatively small proportion of this agent in the atmosphere of crowded localities.