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and, if the eclipse is of short duration, which means a small shadow, the air thirty to forty miles away, where the sun is still shining, sends in a great deal of reflected and refracted light. Usually the hands of a watch can easily be seen; but in the case of a long totality (the greatest possible is 7 minutes 58 seconds), the darkness. is more intense and artificial lights become necessary. There is one other phenomenon, occurring just before totality, which is still unexplained. On every white surface, just before the sun is completely covered, are seen rippling, shadow-like bands, which come and go. What causes them is still a mystery.

The observations which are of importance, and which can be made only during a total eclipse, are, according to Young:

(a) Times of four contacts, and direction of the line joining the cusps of the partially eclipsed sun. These determine with extreme accuracy the relative positions of sun and moon at the moment.

(b) Search for intra-Mercurial planets.

(c) Certain peculiar dark fringes-the socalled "shadow-bands"-which appear upon the surface of the earth for about a minute before and after totality.

(d) Photographic measurement of the intensity of the light at different stages of the eclipse.

(e) Telescopic observations of the details of the prominences and the corona.

(f) Spectroscopic observations, both visual and photographic, upon the "flash spectrum" and upon the spectra of the lower atmosphere of the sun, the prominences and the corona.

(g) Observations with the polariscope upon the polarization of the light of the corona. (h) Drawings and photographic pictures of the corona and prominences.

(i) Miscellaneous observations upon meteorological changes during the progress of the eclipse-barometer, thermometer, wind, etc.and the effects upon the magnetic elements.

To these must be added the photographic work now done with all instruments; also the most recent project, that of photographing the corona and streamers with the three-color process, to obtain, as nearly as possible, a faithful rendering of the colors just as they appeared in the phenomenon itself.

Government Expeditions

Inasmuch as the track of totality may be expected to strike any one place only about once in every 360 years, it is obvious, if we wish to study total eclipses of the sun, that we must go after them. Sending out eclipse expeditions is a recognized feature of astronomical work, and many large observatories are busily at work preparing for the eclipse which will be visible to certain parts of the earth on August 29 and 30, 1905. A glance at the accompanying map will

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A 65-FOOT CAMERA, WITH COLOSTAT, SHOWING LENS AND REFLECTOR. Erected on grounds of U. S. Naval Observatory.-Being tested by Mr. Pelus, Photographer of the Observatory.

show the limits of the eclipse as visible to the earth, and the path of totality. This path goes through the lower part of Canada, across the Atlantic, through Spain, the Mediterranean, the upper part of Africa, across the Red Sea, and across Arabia. Observations must be conducted in the center of the line of totality, in order to get as much totality as possible. To be most effective, they must be conducted as nearly as possible at noon, which means the center of the track from end to end, in order to minimize the atmospheric effects which interfere more or less with all astronomical observations. Consequently, in this particular eclipse, Spain and Northern Africa are indicated as the best regions in which to locate parties. Here the United States Observatory will locate three parties equipped with the very best set of instruments for photographic observations of an eclipse ever carried into the field for the purpose The Canadian Government is also sending out an official expedition to view the eclipse from some point in the Territory of Ungava.

For months the force at the Observatory at Washington has been busily engaged in preparations for this, the biggest event in the astronomical calendar of work. The reader will understand that astronomical instruments are not bought from a stock-house like a readymade suit of clothes, but are specially designed for the work they have to do, and not infrequently are largely made by those who design them. Of the instruments to be used by the three parties in the present case, Mr. W. W. Dinwiddie, of the Naval Observatory staff, has designed the three polar axes, the three portable dark rooms, the three portable houses which are the ends of the big stationary cameras, a ten-inch concave-grating spectroscope, a new instrument called the

chronospectrograph," two cœlostats, made by Gaertner, and in addition has worked out the details of much of the remaining apparatus. The expeditions will be equipped, each with a stationary camera from forty to sixty-five feet long, and each with a polar axis, on which will be mounted a camera of from twelve to fifteen feet focal length, and also spectroscopic apparatus. The necessity of having two cameras for each ex

pedition is obvious. The large camera. takes a large picture, and is intended to make plates which shall record chiefly the phenomena nearest the sun. The small cameras, having a wider angle of view, take smaller pictures, and are designed to picture the eclipse as a whole, including the most most lengthy coronal

streamers.

The first party will locate on the central line of the eclipse, near Burgos, Spain. It will have one large camera, with 5-inch lens, 40-foot focus, making pictures on 14 by 17 inch plate; also one polar axis, with 81⁄2-inch lens, 12-foot focus, for 11 by 14 inch plate, and one 6inch Dallmeyer lens of 36-inch focus, both for photographing the extensions of the corona; besides spectroscopic cameras. On the other end of the cœlostat running the big camera, will be a plane-grating spectroscope, with lens of 5-inch aperture and 42-inch focus, for plates 14 by 11⁄2 inches in size. This party is also equipped with one transit of Venus colostat, with parabolic-grating spectroscope.

Party number two will locafe near Valencia, Spain. This party has the giant camera of this expedition, of 65foot focal length, equipped with a 71⁄2inch triple achromatic lens specially made for this work by Brashear, after curves computed by Prof. C. S. Hastings of Yale. It makes an image of the sun 7 inches in diameter. Besides this monster instrument, shown in one of the illustrations, the party will have a polar axis, with a camera in which is a 6-inch lens of 104-inch focal length, making pictures on an 8 by 10 plate, for use with a color screen; also a 21-foot concave-grating spectroscope, used directly, on the other end of the axis of the colostat.

The third party goes to Sauk Ahauras, Morocco, and is equipped with a camera of 40-foot focus, 5-inch lens; a polar axis, on which is a camera with a 93inch lens of 14-foot focus, for II by 14 inch plates; and a 10-foot concave-grating spectroscope, pointed directly at the sun. This party will also use Mr. Dinwiddie's new chronospectrograph, an instrument designed to give a continuous photographic record of the spectrum of the eclipse, with the time of all the exposures automatically charted; and in ad

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MR. W. W. DINWIDDIE OF THE U. S. NAVAL OBSERVATORY STAFF, TESTING HIS NEW COELOSTAT MIRROR ON A SMALL POLAR AXIS.

and each party has an outfit of dummy glass plates, with ground edges to prevent danger to the hands, so that the man in the camera can learn to put a plate in and get it out of the holder with the greatest possible speed. The plates in these big cameras are handled naked, the plate end of the camera being a little house in which the photographer works. These little houses, like the dark rooms, were designed by Mr. Dinwiddie and go together and come apart without the use of a nail.

What is to be Hoped For?

It is a little difficult to say exactly what is hoped for as a result of these expeditions, inasmuch as the average lay mind does not regard anything less than the finding of a new planet as an astronomical discovery worth noting. Astronomers know, however, that it is from the vast amount of detail collected from different sources apparently unrelated, that

the greatest strides in discovery are made. These and other similar expeditions may not add any new discovery appreciable as such by the public (such as the photographic discovery of an intraMercurial planet, for instance); but if the weather is fair and no hitches occur, the result should be a set of photographs which, when studied, will add largely to our all too small stock of information as to what our sun is, and what causes so many of the inexplicable things that happen upon him and that he causes upon the earth. For, while we know a lot about him, when it is considered that he is some ninety odd millions of miles away, still, what we know is infinitely little compared with what there is to know and what we want to find out. Such expeditions as this, if successful, are better calculated to answer a few of our questions than years of ordinary observation. Let us wish the expeditions all possible

success.

Marvels of Inventive Ingenuity and Mechanical Skill

T

By FRITZ MORRIS

Special Correspondent, THE TECHNICAL WORLD MAGAZINE

HERE is something irresistible, something fascinating in the charm of a timepiece-be it watch, clock, or sundial-for all classes, and for all ages. The clocks of to-day, marvels though they are of mechanical or electrical skill, are no more ingenious than were those of two or three centuries ago.

Ancient Timepieces

The development of mechanical devices to mark time makes an interesting chapter in human history. Probably the most ancient method of measuring the subdivisions of the day was by means of the sundial. This was followed by the water-clock, which was in use in Egypt, Judæa, Babylon, Chaldæa, and Phoenicia. The first water-clock of which history tells us consisted of a basin of water exposed in some corner of a public place. At the extreme end of this vessel there

was a spout from which the liquid flowed, drop by drop, into a receiver having graduations for indicating the hour of the day or night. The water-clock was in use until the seventeenth century. In Japan and China to this day is used a primitive measuring apparatus consisting of a wick, about two feet in length, which takes a certain period to consume. Knots are tied at intervals, which give an indication of the time. The sandglass is also of antiquity; but to this day a sandglass. is used in the British House of Commons to measure certain intervals.

Invention of the Watch

About the year 1500, Peter Henlein, a young member of the Locksmith's Guild of Nuremberg, Bavaria, was engaging himself in improving the timepieces of that period, and he was vexed at the fact that the wheel clocks, the only ones then in existence, which depended for their

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