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tion which have no effect upon our eyes. it was called “the bolometer.” To give We may form a new conception of the any idea of this invention we shall have flames imagined by Milton, that give forth to drag our readers over the corners of a “no light, but rather darkness visible. diagram, but it shall be done as gently as
The normal spectrum spreads out in possible. To begin with, a galvanometer regular proportion the red end and also is required-a contrivance which, when the invisible rays beyond the red. To electricity passes through it, shows the make a chart of this unseen portion was strength of the current by the movement part of the task which Professor Langley of a pointer on a dial. The galvanometer undertook. He proceeded to measure and may be supposed to be placed in the midmap the heat of the entire spectrum. Mel- dle of an arrangement of wires, named, loni was the first to show that heat may in honor of its inventor, “Wheatstone's vary in quality as light does in color. The bridge.” This in the diagram is diamondAllegheny experiments led early to the shaped, but in conclusion that there are not only as many practice may kinds of “heat” as there are colors of have any form, light, but in fact a far greater number so long as the and variety, because of a much more ex- arms are equal: tended range. Perhaps there are animals there are capable of perceiving differences of quali- wires at the juncty akin to color in heat of varying wave- tions of the arms lengths; to us there is only one kind of that enter into difference--that of degree.
the galvanomeIn experiments made by Dr. John W. ter. A battery is Draper the heat in the rays spread out by connected with a grating had been found too faint for the bridge. The measurement, except by concentrating all result of this that fall in half or more than half of the arrangement is
WHEATSTONE'S BRIDGE. spectrum. To carry out the research pro- that if the curposed at Allegheny, to ascertain the tem- rent passing through the arms of the perature for each wave-length, required bridge be of equal strength in each, the something that would measure very mi- galvanometer pointer does not move, benute degrees of heat indeed. For four cause the forces are balanced; but if the years Professor Langley, in the intervals current be stronger on one side, the pointer of other work, strove to do this with ther- will indicate the difference of strength by mo-piles and like contrivances. At last, a proportionate movement over the dial. in sheer desperation, he was compelled to Wlien a metal is heated, it offers increased invent a new instrument. Nearly a year resistance to the passage of the electric was spent in constructing, testing, and current. Hence if only one of the arms perfecting; in overcoming a thousand dif- of the bridge were heated while the curficulties and discouragements; in supply- rent was passing, the pointer would move ing the various "precautions taught by in proportion to the increment of electrical a rather long and painful experience, resistance on that side, and thus indicate which, no doubt, proved the best of teach- the amount of heat absorbed by the wire.
A portion of the income from a fund To obtain more delicate indications, part left by Count Rumford in charge of the of each arm of the bridge was made of National Academy of Sciences was appro- thin strips of metal. This was the first priated to help this work at the observa- step toward a bolometer. For conventory. The liberal citizen of Pittsburgh ience each arm was extended by a loop of who has been before referred to defrayed wire, and the extremity of each loop was the heavy cost of various materials used in made of strips of metal united alternately experiment and construction, and also of at their edges like a Jacob's ladder. The pieces of fine mechanism that had to be two bunches of strips were then brought purchased. He will be most pleased if his together (but not in electrical contact), name is not mentioned here, though to and were put into a cylindrical wooden give it would be fulfilling the principle of case to protect them from accidental heat, the greatest happiness to the greatest num- such as would be given in handling or by ber.
ordinary currents of air. Various imAt last the instrument was finished, and provements were made. It was found
best to split the bunch of strips belonging | but even a specific tariff that might satisfy to one arm into halves, and place the Pennsylvania would not bear heavily on other whole bunch between the halves; sheet-iron strips from one-fifteen-thouthis brought the undivided bunch into the sandth to one-eighteen-thousandth of an
inch thick, of which it may be as truly said as of New Jersey mosquitoes, “ many of them will weigh a pound."
A bolometer axis of the cylinder. A hard rubber tube, I could be constructed, Professor Langley of about the size of a roll of music, was asserts, which would be far more sensithen substituted for the wooden cylinder. tive than the one described, but it would The wires were brought in at one end of not measure heat so accurately. Most the tube, their loops ending in metallic people will be satisfied, however, with strips at its middle, and its other end be- the present instrument, which will give ing open. Then a lid, with a hole in the with exactness a change of temperature centre, capped the open end, the construc- of a ten-thousandth of a degree Centition being such that the hole could be grade, and show some effect from a oneenlarged or diminished. Sundry “dia- hundred - thousandth. At times during phragms” (circles of card-board, each hav- experiments clouds of invisible vapor are ing a hole in the middle) were inserted seen by the eye of the bolometer, and in the open end of the tube at intervals they record their passage occasionally in in the space between the cap and the the brightest and purest sunshine. Of bunches of strips, the object being to cut these clouds no other instrument bears off undesirable heat. The bolometer thus such distinct witness. Quickness is one constructed was set to work by starting of the bolometric virtues. Ten seconds a suitable current from the battery, and are enough for displaying the heat in placing the tube so that the heat to be certain ultra-violet rays. If a two-pound examined should go straight along its lump of ice at freezing-point should get axis to the central bunch of metallic only the warmth of those same rays steadstrips.
ily for a thousand years, it would scarcely Then came a long series of experiments be melted. The last statement will have with strips of different metals under va- I to be taken on trust, as the writer had not rious conditions. The tests included gold-leaf gummed on glass, gold-foil, platinum-foil, various grades of platinum wire, including some only a thousandth of a centimeter in thickness, extremely thin sheet-iron, steel, and finally palladium. The difference between bright metals and those obscured by camphor smoke was also noted. The strips were about the length and width of an ordinary paring from a thumbnail, but thinner than the lightest tissue-paper. Good results were obtained from strips one centimeter long, one millimeter wide, and from a hundredth to a five-hundredth of a millimeter in thickness. An excellent instrument was made with platinum, rolled by Tiffany and Co., of a thickness, or rather time to stay at Allegheny and see it veria thinness, according to the measurements fied. of Professor 0. N. Rood, of less than one- In bolometer experiments no lenses contwenty-five-thousandth of an inch. Most centrate the sunlight. It is taken au naof the metals were rolled in this country, turel, and reflected where it is wanted by
THE BOLOMETER IN ELECTRIC CIRCUIT.
means of a mirror moved by clock-work. By the turning of a screw the whole The contrivance is a modification of the spectrum can be slowly shifted along betrick of the small boy who with a piece fore the eye of the bolometer. When the of looking-glass throws a blinding flash experiment begins, however, the instruof sunshine into a by-passer's eyes. For ment is blind, because a small sliding the purposes of science the beam of re- shutter has been dropped across the slit in flected light is required to shine steadily the wall, cutting off the sunbeam. At a at one spot, where it can pass through a signal the shutter rises; almost instantly hole in the wall of the laboratory. The the image of the galvanometer needle flies hole is a slit whose width can be regula- along the scale, and quivers before the figted, so that, if needed, the line of light | ure that märks the temperature of that which it admits may be reduced to the particular ray which is entering the bomerest thread. After passing through the lometer. The shutter is dropped, the neeslit, the light strikes directly on the grat- dle flies back to its starting-point, and we ing, placed at the farther end of the labo- are ready for another observation. Let ratory, and is reflected back at an angle, us suppose we are examining a certain and spreads out in the beautiful colors of part of the spectrum; for instance, the vithe spectrum. The angle, as before ex- cinity of the B line. The shutter being plained, furnishes a measure of the wave raised, the temperature of the part of the length. The bolometer is placed with its spectrum examined is shown by a moveopen end toward the grating, so as to ment of the needle of three hundred points catch the small portion of the spectrum on the scale. Closing the shutter, perhaps which is to be examined. The electrical we turn the screw a very little, and repeat apparatus is connected with the bolom the experiment with the same result. But eter, and then, after everything is proper- another turn of the screw brings the B ly adjusted, we may proceed to business. line into the eye of the bolometer, and But it should be mentioned that so much then when the shutter is raised, the neecare has to be spent upon adjustments of dle only moves ten points instead of three electrical and other apparatus that if one hundred. person undertook to get ready without as- The most striking of these experiments sistance, his experiment would not begin are in the invisible end of the spectrum, much before sundown.
far beyond the red rays. There, where Vapors of metals inconceivably hot, yet the keenest eye sees no light, and the best cooler than the mass of the sun beneath thermometer, or even thermo-pile, finds no them, floating on the solar surface, inter- heat, the bolometer displays the effect of cept certain parts of the sunbeam, and solar radiation distinctly. More than this; cast, as it were, shadows in their place. it tells of invisible “shadows,” spectral The dark lines of the spectrum are these lines among these viewless rays, and shadows-mere shades of semi-transpar- measures in inches the heat they interent vapor, ninety-two million miles away. cept. It is as if one should see the ghost Incredible as it may seem, the slight loss of a ghost. Here lies the field for future of heat which the "shadows" cause can be discoveries, when we shall ascertain what
measured. The in- are the substances whose vapors make
beam-a thickness lit- cautions that have to be observed in makORIGIN OF FRAUNHOFER tle greater than the ing these experiments; of the various ways
spectral line. The de- in which they have been repeated with
monstration, as wit- different tests and surroundings; of the nessed by the writer, is complete and con- nice reading of scales and position angles vincing. A kind of magic lantern throws with microscopes; of the calculations enan enlarged image of the galvanometer tailed by allowing for absorptions and upon a ground-glass screen, and the move other contingencies. The magician of toments of the pointer are thus made to ap- day does not cry “Open, sesame!" to the pear in inches instead of in hair-breadths. I rock that conceals nature's mysteries; he
strives to dislodge it by long and patient cured for the proposed expedition. Again toil.
the public-spirited citizen of Pittsburgh Already we have some results. Since gave the most essential aid by defraying the time of Newton it has been assumed the entire cost of the special instruments that all the radiations of the sun were to and apparatus required. The Pennsylvabe found in the spectrum, and that these nia Railroad provided transportation for reunited make white light. There is also the explorers and their cumbrous equipa tacit assumption that white light is pure / ments in a Pullman car. Let us never sunlight. Some of the early experiments again hear the taunt of “soulless corporaat Allegheny have been mentioned which tions." showed that the light as we receive it has Mount Whitney, in the Sierra Nevada been somewhat altered by the sun's atmos- of Southern California, was selected as phere. The change is an absorption of the goal of promise. This choice was rays from the blue end of the spectrum. made after conferring with officers of the It follows that our sunlight is more red army and of the Coast Survey who were and less blue, and far less intense, than it familiar with the Western wilderness. would be if the solar atmosphere did not The mountain rises nearly to the height intervene. But we are concerned with of Mont Blanc. It is in one of the most something nearer home. Our own atmos- arid regions on the globe. So steep is it phere repeats the performance, strangles that two stations can be placed upon it many rays at the blue end of the spectrum, within easy signaling distance of each othand comparatively few at the red end. er, but differing in elevation by more than What does this mean? Let us shake two miles. Little was known beforehand hands with our friend who wears green as to the possibility of carrying the bulky goggles. We too have all our lives seen implements of science up the mountain. things in a false light. If we could place Through the representations of General ourselves outside our atmosphere-say on Hazen, a small military escort, under the the moon-we should find that sunlight command of Captain Michaelis, U.S.A., is not white; that the sun itself is really was provided, both as a measure of securiblue. To the inhabitants of “other worldsty and, in case of need, to assist in transthan ours" the sun may be a bluer star portation. than Vega.
Four of the exploring party started At Allegheny the series of experiments from Pittsburgh last summer, and were leading to such a result consisted chiefly joined at San Francisco by three others in comparing the rays from the mid-day and the escort. A point about four hunsun with those received when the orb ap-dred miles farther south was reached by proached the horizon. But though the railway travel. At Caliente the comforts experiments were conducted in winter- of the Pullman car were left behind, and our driest season-they left some uncer- the party began a slow march across the tainty on two points-the effect of moisture Inyo desert. For one hundred and twenin the air, and the question whether the ty miles this part of the route was shadeabsorption of blue rays was in like propor-less and waterless. They passed by Death's tion to thickness of atmosphere at greater Valley, two hundred feet below sea-level, heights. To decide these and similar where, only a few years ago, an entire questions it was deemed advisable to emigrant train perished miserably. The make an expedition to the driest place mountain range shuts this district from and the highest mountain in the United the rest of California. When a star route States.
was established to carry the mail from Before we can attain much accuracy in Caliente through this valley, water had one of the most important of modern to be hauled twenty miles to the stage studies-weather science-we must at all stations. events learn more on three points: the to- A camp was pitched at the foot of the tal heat of the sun, so as to know if it va- Sierras, near a place called Lone Pine. It ries; the amount of absorption by dry and is a fair specimen of South western frontier moist air respectively; the kinds of heat towns, and consists chiefly of one street. absorbed. The value of this class of re- When a lucky party of Mexicans are vissearches was recognized by General Hazen, iting the town, playing-cards thrown out of the United States Signal Service, and of the windows litter the street as with the help of the War Department was se autumn leaves. Spasms of morality are felt at intervals in Lone Pine, social earth- | possible to carry the apparatus up the quakes, when superfluous gamblers must nearest side. To reach the desired point get out or be wiped out. The tone of so- a détour had to be made around to the ciety is better than in some towns farther farther side of the mountain. Seven or down the border - Parsontown, for in- eight days were thus consumed. Patient stance, which is famous for its three suc- mules carried their valuable burdens of cessive clergymen. The first of these siderostats, telescopes, bolometers, actishepherds shot the husband of one of his nometers, pyrheliometers, and the like, flock, and married the widow. The sec-up stony heights and around sharp peaks, ond was partner in a faro bank, and lost without serious mishap. The mountain influence because his“ pard" cheated. has been rarely if ever before ascended; The third, in addition to his religious du- certainly never with such luggage. ties at Parsontown, kept one of the stage As the party went higher the air grew stations on the star route. He had his colder and the sunshine hotter. Those enemies; one of them came fooling around men of science had been tanned by weeks the station, and was shot by the preacher. of exposure in the desert, with the shaded So far there was nothing startling. But thermometer at 110° F. Yet after they provisions were scarce, and the clergy- reached the level of perpetual snow, the man fed stage-passengers on broiled and sun's rays burned their hands and faces jerked enemy for several days. An army anew and very severely. Most of the surgeon happened that way, dined on a party looked as though they had been cutlet, and recognized one of the bones as scorched by fire. It is worth noting that human. Clergyman No. 3 fled; he was this was not caused by reflection from afterward captured, and suspended at once snow, to which similar experience of Alfrom the ministry and a pine branch. pine climbers has been attributed. The Parsontown has now no regular preach- party on Mount Whitney received their ing.
worst sunburn when travelling over bare The camp near Lone Pine was organ- rocks, though isolated snow - fields lay ized for continual use as a low-level sta- above and below. tion during the stay of the scientific party. A camp was pitched at an altitude of Numerous observations were to be taken 13,000 feet, and the heavier apparatus was there and on the mountain simultaneously placed there. The peak rises 2000 feet each day, and in different parts of the day. higher, and was climbed every day by obThe Lone Pine station being far below, servers carrying portable instruments. its observations would show the effect, as The side of the peak nearest the camp was compared with those of the mountain-top, an almost vertical sheet of dark gray of a very much greater thickness of at- granite, seamed here and there by gullies mosphere.
filled with bowlders that occupied the beds Mount Whitney was in plain sight from of old water-courses. A little streamlet the lower camp; apparently within gun- gleamed at intervals in one of the clefts. shot. When a picture was taken that It was fed from snows above, and served
to mountain seemed to be part of the range to make a small swampy meadow and a of peaks in the foreground, or at farthest pond, on the ledge selected for the camp. a little back of them; in fact it was a long The pond has been dignified, in Western way behind, and at least four thousand vernacular, with the title of a “lake.” feet higher than the intervening peaks. At such elevations, with only tents for Patches of white on its gray and jagged shelter, with high winds and cold and outline were found to be snow; and what mountain sickness, there was little comseemed a coating of moss turned into fort and some trouble in making the nice broad forests when the telescope was and numerous observations laid down in brought to bear. The deceptive effect the programme. Accidents will happen, was caused by the extreme dryness and too, even to parties in the highest posipurity of the atmosphere, and the absence tions. An excellent telescope, kindly of what artists call “aerial perspective.” | lent by the astronomer of Harvard UniWe ordinarily judge of the distance of an versity, was found (too late for remedy) object in a landscape by its comparative to be suffering from a disorder of its eyedimness. The mountain was really more pieces. A large and very costly mirror, than fifteen miles away.
covered with a face of silver, polished with After some exploration it was found im- exquisite skill, and carefully wrapped,