It was an equally disciplined eye which in the laboratory first noticed that air is non-conducting until traversed by an X-ray, when it becomes conducting in a noteworthy degree. The field of radio-activity, at which we have glanced in this book, owes its cultivation to observers keen to note phenomena utterly unlike those before dwelt upon by the human eye. Often close observers learn what would never be imagined as possible: in rifle-making the tendency of the drills, which revolve nearly a thousand times a minute, to follow the axial line in a revolving bar is a fact which may be accounted for after observation, but which no one would predict.
One day on the Glasgow and Ardrossan Canal a spirited horse took fright; it was then observed, with astonishment, that a boat, the “Raith,” to which it was attached, for all its increased speed, went through the water with less resistance than before. The vessel rode on the summit of a wave of its own creation with this extraordinary effect. The “Raith,” said Mr. Scott Russell, “weighed 10,239 pounds, requiring a force of 112 pounds to drag it at 4.72 miles an hour; 275 pounds at 6.19 miles an hour, and but 2681⁄2 pounds at 10.48 miles per hour.” Thus paradoxically was reversed the rule that the resistance of a vessel increases rapidly as she is moved through the water. Mr. Russell added:—“Some time since a large canal in England was closed against general trade by want of water, drought having reduced the depth from 12 to 5 feet. It was then found that the motion of the light boats was more easy than before; the cause was obvious. The velocity of the wave was so much reduced by the diminished depth, that, instead of remaining behind the wave, the vessels rode on its summit.”
The Mississippi Jetties of James B. Eads.
One of the most difficult problems ever solved by an American engineer was the making navigation safe for vessels of fairly deep draft in the lower branches of the Mississippi. The difficulties were overcome by James B. Eads, of St. Louis, in his system of jetties. He remarked, says his biographer, Mr. Louis How, that other things being equal, the amount of sediment which a river can carry is in direct proportion to its velocity. When, for any reason, the current becomes slower at any special place, it drops part of its burden of sediment at that place, and when it becomes faster again it picks up more. Now, one thing that makes a river slower is an increase of its width, because then there is more frictional surface; and contrariwise, one of the things that makes it faster is a decrease of its width. Narrow the Mississippi then, at its mouth, said Eads, and it will become swifter there, and consequently will remove its soft bottom by picking up the sediment (of which it will then hold much more), and by carrying it out to the gulf, to be lost in deep water and swept away by currents, you will have your deep channel. In other words, if you give the river some assistance by keeping its current together, it will do all the necessary labor and scour out its own bottom. This sound reasoning, based upon observation as sound, was duly embodied in a series of jetties which have proved successful.
Observation Suggests an Experiment.
Such a river as the Mississippi taking its source through an alluvial plain, has bends which go on increasing by the wearing away of the outer banks, and the deposition of mud, sand and gravel on the inner bank. In 1876 at the Glasgow meeting of the British Association for the Advancement of Science, Professor James Thomson showed a model which made the phenomena of the case perfectly clear. A stream eight inches wide and less than two inches deep, flowed round a bend. As it turned this bend the water exerted centrifugal force, while a thin layer of the water at the bottom, representing a similar layer close to a river-bed, was retarded by its friction with the remainder of the stream, exerting less centrifugal force than like portions of the larger body of water flowing over it farther away from the bottom. Consequently the bottom layer flowed in obliquely across the channel toward the inner bank; rising up in its retarded motion betwixt the fast flowing water it protected the inner bank from scour. At the same time this retarded current brought with it sand and other detritus from the bottom, duly deposited along the inner bank of the stream.
Instrumental Aids to Observation.
The powers of the eye, acute as they are, have narrow limits; inestimable therefore is the value of the microscope, the telescope and the camera which bring to view uncounted images otherwise unseen. Let us remark how in the early days of instrumental aids a great observer just missed noting a phenomenon of utmost importance,—the black lines of the solar spectrum, upon which Fraunhofer, an optician of Munich, based his spectroscope. In sending a solar beam through a lens and a prism Sir Isaac Newton admitted the rays through an oblong slit at times as narrow as one twentieth of an inch. He saw the familiar colors, from red to violet, and nothing more. Even with a crown lens, such as he probably used, four lines distinctly appear; that is, they appear to-day, to an observer who is looking for them. In 1802 these lines were observed, as far as we know, for the first time on record, by Dr. Wollaston, who drew six of them in a diagram accompanying a paper in the Philosophical Transactions. Four of these lines he regarded as boundaries of the colors of the spectrum; of the other two lines he attempted no explanation. He used prisms of various materials but found no alteration in the lines while he studied a sunbeam. When he employed candles or an electric light he found the appearances different, why, he could not undertake to explain. In 1814, Fraunhofer observed these lines in detail, mapped them, and proved that they identified elements long known to chemists. As he built his spectroscope he gave the chemist, the physicist and the astronomer an instrument of research worthy a place beside either the microscope or the telescope.
Dr. Wollaston, in 1802, as we have seen stood upon the threshold of spectroscopy without knowing it. During the same year he performed an experiment which took him into the field of photography without his recognizing the possibilities of that wonderful art. He took paper which had been dipped in muriate of silver and caught on its surface impressions of the ultra-violet light in a solar spectrum. These rays, as rings, were reflected from a thin plate of air, as in the case of the colors of thin plates, at distances corresponding to their proper places in the spectrum. Thus was established the close analogy between rays visible and invisible, and by a method destined to give mankind a universal limner in light of all kinds, and in much radiance which is not luminous at all.