This is substantially true, but it must be taken in connection with other facts which have a far greater influence on a balloon's position or motion.

For instance, in the volume just referred to it is stated by M.

Gaston Tissandier that on one occasion when aloft he threw overboard a chicken bone, and, immediately consulting a barometer, had to admit on "clearest evidence that the bone had caused a rise of from twenty to thirty yards, so delicately is a balloon equipoised in the air." Here, without pausing to calculate whether the discharge of an ounce or so would suffice to cause a large balloon to ascend through ninety feet, it may be pointed out that the record cannot be trustworthy, from the mere fact that a free balloon is from moment to moment being subjected to other potent influences, which necessarily affect its position in space. In daytime the sun's influence is an all-important factor, and whether shining brightly or partially hidden by clouds, a slight difference in obscuration will have a ready and marked effect on the balloon's altitude. Again, a balloon in transit may pass almost momentarily from a warmer layer of air to a colder, or vice versa, the plane of demarcation between the two being very definite and abrupt, and in this case altitude is at once affected; or, yet again, there are the descending and ascending currents, met with constantly and unexpectedly, which have to be reckoned with.

Thus it becomes a fact that a balloon's vertical course is subjected to constant checks and vicissitudes from a variety of causes, and these will have to be duly borne in mind when we are confronted with the often surprising results and readings which are supplied by scientific observers. With regard to the close proximity, without appreciable intermingling, of widely differing currents, it should be mentioned that explorers have found in regions where winds of different directions pass each other that one air stream appears actually to drag against the surface of the other, as though admitting no interspace where the streams might mingle. Indeed, trustworthy observers have stated that even a hurricane can rage over a tranquil atmosphere with a sharply defined surface of demarcation between calm and storm. Thus, to quote the actual words of Charles Darwin, than whom it is impossible to adduce a more careful witness, we find him recording how on mountain heights he met with winds turbulent and unconfined, yet holding courses "like rivers within their beds."