could be seen, the lower part of the column being altogether concealed from view. Using the same barometer a few months afterwards, in the mountains of Jamaica, I was not a little surprised to find that the barometer on one occasion stood higher than it had done when examined three or four hundred feet lower down. Immediately suspecting the cause, and having a fine turnscrew with me, I detached the case altogether from the tube and cistern, and then perceived that a separation had taken place in the column of mercury, low down in the tube, arising doubtless from a small quantity of air which had made its way in, in spite of the usual precautions. It was of course soon tapped out, and the barometer being replaced in its case, gave its proper indication. After this experience, on my return to England, I had a barometer made in which the case was so arranged as to permit the whole column of mercury to be inspected, from the cistern upwards, at every observation; and I have always used it since.
"Now it is certainly possible that a similar occurrence may have taken place on Pico Ruivo. It is true that on the return to Funchal the barometer agreed with its indication at the same place before our departure; but this does by no means preclude the possibility that the column may not have been perfect on the Peak. The escape of air in travelling might be more easy than its introduction. It may be a very rare occurrence when the proper precautions are not omitted, and I dare say it is so. I do not remember an instance of the kind taking place in the barometer which I have since used, and in which I always examine the whole column. But it would appear undoubtedly preferable to have portable barometers so constructed as always to admit of this examination.
