The effects on health of changes of weather preceding rain have long been known, and since the discovery of the atmos pheric pressure, and of the instrument called the barometer, with which that pressure is measured, the idea that there is a close connection between a fall of the barometer and certain painful affections, like neuralgia and rheumatic pains, has become gen eral. In our day this subject has been remarkably illustrated, on one side of it, by the observations which have been scientifically made .on the effects of pressure upon men who are obliged to work in compressed air, and who suffer from the affection de scribed at p. 392-4 as caisson, or coffer-dam disease. The subject has also been studied on the other side by observations on the effects of reduced pressure upon persons who are ascending moun tains, or upon those who are rising, in a balloon, into a higher and rarefied atmosphere.
The influence of atmospheric pressure, which I have briefly described in the pages above referred to, is variable according to the susceptibilities of the persons who are subjected to it. In fact, there is nothing which shows more conspicuously the differ ence of human susceptibilities to external influences than the pressure of air. Dr. Andrew Smith, in his record of the phe nomena he noticed in the caisson of the East River Bridge at New York, found that a large proportion of the men bore the excessive pressure, reaching at last to thirty-six additional pounds to the square inch, without the slightest ill effect; whilst in others some severe symptoms resulted from a very short ex posure to slight pressure. In one instance a student of engineer ing, after a brief stay under an additional pressure not exceeding fifteen pounds, was seized on coming into the air under ordinary pressure, with temporary paralysis. The difference, Dr. Smith thinks, lies in the difference of efficiency of the vaso-motor system, or perhaps in the structure of the blood-vessels themselves, so that in one case the vessels resume at once their natural condition when the pressure is removed, while in the other case the abnor mal distribution of the blood persists in certain parts of the vas cular surface. " Whatever," he argues, " the predisposing condi tion may be in this latter class of cases, we have only to assume its existence in an exaggerated degree to bring the subject within the range of ordinary barometrical changes. If one man can bear a change of ninety inches, and come out of it to the ordinary pressure without feeling it, while another is paralyzed by a change from thirty inches of pressure to the ordinary, it is not incredible that a third may have aching limbs as a result of being subjected to a fall of two inches of pressure." This is the reasoning suggested by Dr. Smith, and from these inferences, as well as from many others which he has drawn from his unrivalled field of research, he explains why some cicatrices, — scars, such as those following gunshot wounds, are specially prone to be subjected to neuralgiac pains during change of weather. He also explains why, on bright days, when the air is light, and the barometer is high, when, that is to say, the greater atmospheric pressure on the surface of the body drives the blood into the interior of the body, and especially into the organs in closed cavities, such as the brain, the mind is active, and the muscles vigorous; while on damp, foggy days, mental effort is irk some, the limbs drag, the appetite is small, and the whole tone of the system lowered.
The changes are more distinctive still in those who are prone to disease, and in whom the vessels are already enfeebled or changed in structure. There can be no doubt that the effects of reduced pressure tell with intense force on large populations who are already enfeebled.
I have dwelt on variation of the atmospheric pressure as a cause of disease, on account of its great practical meaning. It embraces so much. Whether the variation from high to low pressure ever acts as a primary cause of disease it is impossible to define, bat that it acts as an exciting cause, of the most serious character, on persons predisposed to disease there can be no doubt whatever. A sudden reduction of the atmospheric pressure, which means a fall in the barometer, is attended with risk of pressure of blood, leading to pressure on nervous fibre and production of pain; to internal congestion; to fever; to apoplexy : an increased pressure, which means a rise in the barometer, is at tended with greater risk of injury from physical or mental shock.
The effect of the variation extends in yet another direction. I have pointed out in an essay on " Meteorology in Relation to Sur gical Practice," that the atmospherical pressure plays an important part in the results of surgical procedures, and that with a falling barometer the chances of success are reduced.
Thus the time is favorable for operation, (a) When the barometer is steadily rising.
(b) When the barometer is steadily high.
(c) When the wet-bulb thermometer shows a reading of five degrees lower than the dry-bulb.
(d) When, with a high barometer and a difference of five de grees in the two thermometers, there is a mean temperature at or above 55° Fahr.
The time is unfavorable for operation, (a) When the barometer is steadily falling.
(b) When the barometer is steadily low.
(c) When the wet-bulb thermometer approaches the dry-bulb within two or three degrees.
(d) When, with a low barometrical pressure and approach to unity of reading of the two thermometers, there is a mean tem perature above 45° and under 55° Fahr.
On the same point Dr. Addenill Ilewson has published in the Pennsylvanian Hospital reports a register of 259 operations with the accompanying meteorological conditions, and the results con nected with the operations. These results, as stated by Dr. Hew son, stand as follows :— " On the occasion of the 259 operations, the barometer was ascending in 102, descending in 123, and stationary in 34.
Fifty-four of the whole number were fatal; 11 of them were operated on when the barometer was ascending, 35 when it was descending, and s when it was stationary.
Of the successful cases, 91 were operated on with an ascend ing barometer, ss with it descending, and 26 with it stationary.
" From which it would seem," says Dr. Hewson, " that we got a mortality, when the operation was performed with the barometer ascending, of not quite 11 (10.7) per cent.; of over 20 (20.6) per cent. with it stationary; and over 2s (2s.4) per cent. with it descending. Of the fatal cases, the average length of time which the patient survived the operation was only seven days when the barometer was ascending, and thirteen when it was descending; and of the cases which died within three days over 75 per cent. were when the barometer was ascending."