tion which have no effect upon our eyes. We may form a new conception of the flames imagined by Milton, that give forth "no light, but rather darkness visible."

The normal spectrum spreads out in regular proportion the red end and also the invisible rays beyond the red. To make a chart of this unseen portion was part of the task which Professor Langley undertook. He proceeded to measure and map the heat of the entire spectrum. Melloni was the first to show that heat may vary in quality as light does in color. The Allegheny experiments led early to the conclusion that there are not only as many kinds of "heat" as there are colors of light, but in fact a far greater number and variety, because of a much more extended range. Perhaps there are animals capable of perceiving differences of quality akin to color in heat of varying wavelengths; to us there is only one kind of difference that of degree.

it was called "the bolometer."
To give
any idea of this invention we shall have
to drag our readers over the corners of a
diagram, but it shall be done as gently as
possible. To begin with, a galvanometer
is required-a contrivance which, when
electricity passes through it, shows the
strength of the current by the movement
of a pointer on a dial. The galvanometer
may be supposed to be placed in the mid-
dle of an arrangement of wires, named,
in honor of its inventor, "Wheatstone's
bridge." This in the diagram is diamond-
shaped, but in
practice may
have any form,
so long as the
arms are equal:
there are cross
wires at the junc-
tions of the arms
that enter into

the

galvanometer. A battery is connected with the bridge. The result of this arrangement is that if the cur

WHEATSTONE'S BRIDGE.

bridge be of equal strength in each, the galvanometer pointer does not move, because the forces are balanced; but if the current be stronger on one side, the pointer will indicate the difference of strength by a proportionate movement over the dial. When a metal is heated, it offers increased resistance to the passage of the electric current. Hence if only one of the arms of the bridge were heated while the current was passing, the pointer would move in proportion to the increment of electrical resistance on that side, and thus indicate the amount of heat absorbed by the wire.

In experiments made by Dr. John W. Draper the heat in the rays spread out by a grating had been found too faint for measurement, except by concentrating all that fall in half or more than half of the spectrum. To carry out the research proposed at Allegheny, to ascertain the tem-rent passing through the arms of the perature for each wave-length, required something that would measure very minute degrees of heat indeed. For four years Professor Langley, in the intervals of other work, strove to do this with thermo-piles and like contrivances. At last, in sheer desperation, he was compelled to invent a new instrument. Nearly a year was spent in constructing, testing, and perfecting; in overcoming a thousand difficulties and discouragements; in supplying the various "precautions taught by a rather long and painful experience,' which, no doubt, proved the best of teachers. A portion of the income from a fund left by Count Rumford in charge of the National Academy of Sciences was appropriated to help this work at the observatory. The liberal citizen of Pittsburgh who has been before referred to defrayed the heavy cost of various materials used in experiment and construction, and also of pieces of fine mechanism that had to be purchased. He will be most pleased if his name is not mentioned here, though to give it would be fulfilling the principle of the greatest happiness to the greatest number.

At last the instrument was finished, and

The

To obtain more delicate indications, part of each arm of the bridge was made of thin strips of metal. This was the first step toward a bolometer. For convenience each arm was extended by a loop of wire, and the extremity of each loop was made of strips of metal united alternately at their edges like a Jacob's ladder. two bunches of strips were then brought together (but not in electrical contact), and were put into a cylindrical wooden case to protect them from accidental heat, such as would be given in handling or by ordinary currents of air. Various improvements 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."

THE BOLOMETER.

|

A bolometer

could be constructed, Professor Langley asserts, which would be far more sensitive than the one described, but it would not measure heat so accurately. Most people will be satisfied, however, with the present instrument, which will give with exactness a change of temperature of a ten-thousandth of a degree Centigrade, and show some effect from a one

axis of the cylinder. A hard rubber tube, of about the size of a roll of music, was then substituted for the wooden cylinder. The wires were brought in at one end of the tube, their loops ending in metallic strips at its middle, and its other end being open. Then a lid, with a hole in the centre, capped the open end, the construction being such that the hole could be enlarged or diminished. Sundry "dia-hundred-thousandth. At times during phragms" (circles of card-board, each having a hole in the middle) were inserted in the open end of the tube at intervals in the space between the cap and the bunches of strips, the object being to cut off undesirable heat. The bolometer thus constructed was set to work by starting a suitable current from the battery, and placing the tube so that the heat to be examined should go straight along its axis to the central bunch of metallic strips.

Then came a long series of experiments with strips of different metals under various 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 a thinness, according to the measurements of Professor O. N. Rood, of less than onetwenty-five-thousandth of an inch. Most of the metals were rolled in this country,

Of

experiments clouds of invisible vapor are
seen by the eye of the bolometer, and
they record their passage occasionally in
the brightest and purest sunshine.
these clouds no other instrument bears
such distinct witness. Quickness is one
of the bolometric virtues. Ten seconds
are enough for displaying the heat in
certain ultra-violet rays. If a two-pound
lump of ice at freezing-point should get
only the warmth of those same rays stead-
ily for a thousand years, it would scarcely
be melted. The last statement will have
to be taken on trust, as the writer had not

THE BOLOMETER IN ELECTRIC CIRCUIT.

time to stay at Allegheny and see it verified.

In bolometer experiments no lenses concentrate the sunlight. It is taken au naturel, and reflected where it is wanted by

means of a mirror moved by clock-work. The contrivance is a modification of the trick of the small boy who with a piece of looking-glass throws a blinding flash of sunshine into a by-passer's eyes. For the purposes of science the beam of reflected light is required to shine steadily at one spot, where it can pass through a hole in the wall of the laboratory. The hole is a slit whose width can be regulated, so that, if needed, the line of light which it admits may be reduced to the merest thread. After passing through the slit, the light strikes directly on the grating, placed at the farther end of the laboratory, and is reflected back at an angle, and spreads out in the beautiful colors of the spectrum. The angle, as before explained, furnishes a measure of the wavelength. The bolometer is placed with its open end toward the grating, so as to catch the small portion of the spectrum which is to be examined. The electrical apparatus is connected with the bolom eter, and then, after everything is properly adjusted, we may proceed to business. But it should be mentioned that so much care has to be spent upon adjustments of electrical and other apparatus that if one person undertook to get ready without assistance, his experiment would not begin much before sundown.

By the turning of a screw the whole spectrum can be slowly shifted along before the eye of the bolometer. When the experiment begins, however, the instrument is blind, because a small sliding shutter has been dropped across the slit in the wall, cutting off the sunbeam. At a signal the shutter rises; almost instantly the image of the galvanometer needle flies along the scale, and quivers before the figure that marks the temperature of that particular ray which is entering the bolometer. The shutter is dropped, the needle flies back to its starting-point, and we are ready for another observation. Let us suppose we are examining a certain part of the spectrum; for instance, the vicinity of the B line. The shutter being raised, the temperature of the part of the spectrum examined is shown by a movement of the needle of three hundred points on the scale. Closing the shutter, perhaps we turn the screw a very little, and repeat the experiment with the same result. But another turn of the screw brings the B line into the eye of the bolometer, and then when the shutter is raised, the needle only moves ten points instead of three hundred.

The most striking of these experiments are in the invisible end of the spectrum, far beyond the red rays. There, where the keenest eye sees no light, and the best thermometer, or even thermo-pile, finds no heat, the bolometer displays the effect of solar radiation distinctly. More than this; it tells of invisible "shadows," spectral lines among these viewless rays, and measures in inches the heat they intercept. It is as if one should see the ghost of a ghost. Here lies the field for future discoveries, when we shall ascertain what are the substances whose vapors make these unseen shadows. Meanwhile let us not be unhappy because we can not see the extreme ends of the spectrum, for who would wish to be as sensitive as a bolometer?

Vapors of metals inconceivably hot, yet cooler than the mass of the sun beneath them, floating on the solar surface, intercept certain parts of the sunbeam, and cast, as it were, shadows in their place. The dark lines of the spectrum are these shadows-mere shades of semi-transparent vapor, ninety-two million miles away. Incredible as it may seem, the slight loss of heat which the "shadows" cause can be measured. The instrument which performs this marvel is called the "linear" bolometer; it presents only the edge of a metallic strip to the sunLittle has been said of the many prebeam--a thickness lit- cautions that have to be observed in makORIGIN OF FRAUNHOFER the 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. | rock that conceals nature's mysteries; he

LINE.

strives to dislodge it by long and patient | cured for the proposed expedition. Again toil. the public-spirited citizen of Pittsburgh gave the most essential aid by defraying the entire cost of the special instruments and apparatus required. The Pennsylvania Railroad provided transportation for the explorers and their cumbrous equipments in a Pullman car. Let us never again hear the taunt of "soulless corporations."

Mount Whitney, in the Sierra Nevada of Southern California, was selected as the goal of promise. This choice was made after conferring with officers of the army and of the Coast Survey who were familiar with the Western wilderness. The mountain rises nearly to the height of Mont Blanc. It is in one of the most

Already we have some results. Since the time of Newton it has been assumed that all the radiations of the sun were to be found in the spectrum, and that these reunited make white light. There is also a tacit assumption that white light is pure sunlight. Some of the early experiments at Allegheny have been mentioned which showed that the light as we receive it has been somewhat altered by the sun's atmosphere. The change is an absorption of rays from the blue end of the spectrum. It follows that our sunlight is more red and less blue, and far less intense, than it would be if the solar atmosphere did not intervene. But we are concerned with something nearer home. Our own atmos-arid regions on the globe. So steep is it phere repeats the performance, strangles many rays at the blue end of the spectrum, and comparatively few at the red end. What does this mean? Let us shake hands with our friend who wears green goggles. We too have all our lives seen things in a false light. If we could place ourselves outside our atmosphere-say on the moon-we should find that sunlight is not white; that the sun itself is really blue. To the inhabitants of "other worlds than ours" the sun may be a bluer star than Vega.

that two stations can be placed upon it within easy signaling distance of each other, but differing in elevation by more than two miles. Little was known beforehand as to the possibility of carrying the bulky implements of science up the mountain. Through the representations of General Hazen, a small military escort, under the command of Captain Michaelis, U.S.A., was provided, both as a measure of security and, in case of need, to assist in transportation.

Four of the exploring party started from Pittsburgh last summer, and were joined at San Francisco by three others and the escort. A point about four hun

At Allegheny the series of experiments leading to such a result consisted chiefly in comparing the rays from the mid-day sun with those received when the orb ap-dred miles farther south was reached by proached the horizon. But though the experiments were conducted in winterour driest season-they left some uncertainty on two points-the effect of moisture in the air, and the question whether the absorption of blue rays was in like proportion to thickness of atmosphere at greater heights. To decide these and similar questions it was deemed advisable to make an expedition to the driest place and the highest mountain in the United States.

Before we can attain much accuracy in one of the most important of modern studies-weather science-we must at all events learn more on three points: the total heat of the sun, so as to know if it varies; the amount of absorption by dry and moist air respectively; the kinds of heat absorbed. The value of this class of researches was recognized by General Hazen, of the United States Signal Service, and the help of the War Department was se

railway travel. At Caliente the comforts of the Pullman car were left behind, and the party began a slow march across the Inyo desert. For one hundred and twenty miles this part of the route was shadeless and waterless. They passed by Death's Valley, two hundred feet below sea-level, where, only a few years ago, an entire emigrant train perished miserably. The mountain range shuts this district from the rest of California. When a star route was established to carry the mail from Caliente through this valley, water had to be hauled twenty miles to the stage stations.

A camp was pitched at the foot of the Sierras, near a place called Lone Pine. It is a fair specimen of Southwestern frontier towns, and consists chiefly of one street. When a lucky party of Mexicans are visiting the town, playing-cards thrown out of the windows litter the street as with autumn leaves. Spasms of morality are

felt at intervals in Lone Pine, social earth- | possible to carry the apparatus up the nearest side. To reach the desired point a détour had to be made around to the farther side of the mountain. Seven or eight days were thus consumed. Patient mules carried their valuable burdens of siderostats, telescopes, bolometers, actinometers, pyrheliometers, and the like, up stony heights and around sharp peaks, without serious mishap. The mountain has been rarely if ever before ascended; certainly never with such luggage.