One of the most interesting applications of the Doppler-Fizeau principle is the determina tion of the rotation periods of the sun and the planets. As the sun rotates the eastern edge moves toward us while the western moves away from us. Measurements of the rotation of the sun have been carried out mainly by Hahn of Edinburgh, Plaskett at Ottawa, and Adams at Mount Wilson. The rotation time at the sun's equator is found to be 24.8 days; at latitude 30°, 26.3 days; at latitude 28.1 days; at latitude 60°, 30.2 days; and at latitude 75°, 31.9 days. Adams used more than 20 lines of iron, titanum, manganese, etc., and he found that the different elements do not all show the same rotation period. The differences, according to Adams, can be explained as being due to differences of level of the layers producing the different lines. Most interesting are the results from the H a line which shows a rotation pe riod at the equator of 24.0 days instead of 24.8, and at latitude 75°, a rotation in 26.3 days in stead of 31.9 days, as shown by the metallic lines at the same latitude. Hale has investigated the rotation time by means of the flocculi. The calcium flocculi show a decrease in rotation speed in higher latitudes, just as is shown by the spots, but in a lesser degree, while on the other hand the hydrogen flocculi H d show an increase from a rotation speed of 252 days at the equator to 24.7 days in middle latitudes. There is still a vast amount of work left to be done before a thorough knowledge is obtained of the manner of rotation of the sun.
. In investigating the rotation of the planets, most satisfactory results are obtained from Jupiter. On account of its great size and rapid rotation, a point oil Jupiter's equator moves with a speed of 12% kilometres per second. If the slit of the spectrograph is placed across the planet, parallel to the equator, the lines of the spectrum will be inclined on account of the great rotational velocity. Although the planet Uranus shows no conspicuous markings, and its rotation time, therefore, cannot be deter mined from visual observations, Lowell and Slipher have been able to show from spectro scopic observations that it rotates once in log hours, and in the retrograde direction, or in the same direction that the satellites move about Uranus. Applied to Venus, the spectro scopic method has not had the same degree of success due to the fact that the linear motion from rotation is small, and, consequently, diffi cult to measure. While Slipher finds the rota tion period to be 225 days, thus corroborating the results obtained visually by Schiaparelli and Lowell, Belopolsky in Pulkowa finds a period of about 24 hours.
Water Vapor on According to Percival Lowell there are a great number of canals on Mars which are for the purpose of irrigating the arid planet. As there is very little water visible on the surface of Mars, Lowell concludes that the atmosphere is rich in water vapor. This is a matter that can be investigated spectroscopically. Since the moon has no atmosphere, it shows the same spectrum as the sun. In coming from the moon, rays of light pass through the earth's atmosphere, so that its spectrum might show atmospheric lines due to the earth's water vapor. When light reaches us from Mars the rays from the sun have twice passed through the atmosphere of Mars. If the spectrum of Mars is compared with that of the moon, care being taken that the two spectra are observed under as nearly as possible identical conditions regarding alti tude, the presence of water vapor in the earth's atmosphere, etc., then if the atmosphere of
Mars is rich in water vapor, these lines should show by increased absorption at the red end of the spectrum. Slipher believes that he has proved this to be the case. Campbell, under vastly superior conditions, observing from the top of Mount Whitney (15,000 feet), finds that the spectra of Mars and moon are identical. Campbell has also tested this matter in a differ ent manner, but with negative results. The water vapor lines resulting from the earth's atmosphere should show a different radial ve locity from those formed by the Martian at mosphere, since Mars is moving. Although the terrestrial atmospheric lines were found and measured by Campbell, there were none found due to Mars.
The Solar Astrophysical work along entirely different lines has been carried out at the Astrophysical Observatory of the Smithsonian Institution for the purpose of de termining the value of the solar constant, or the amount of heat from the sun that reaches the outside of the earth's atmosphere. The amount of heat received at the surface of the earth is measured by means of the pyrheli ometer or actinometer. It is very difficult to eliminate the absorbing effect of the earth's atmosphere. Abbot has shown that this can be accomplished by the use of the bolometer, invented by Langley and used in connection with a sensitive galvanometer. The bolometer and galvanometer, both perfected in the hands of Abbot, are used to measure the energy at different regions of the spectrum of the sun. The determination made by Langley from the top of Mount Whitney placed the value of the solar constant at 3.0 calories. More complete observations have been made by Abbot at Washington at sea-level, at Mount Wilson at an altitude of 5,700 feet, at Bassour in north ern Africa, 3,900 feet, and at Mount Whitney in California at an altitude of 14,500 feet. Ob servations nearly simultaneous have been car ried on at Washington and Mount Wilson, and also at Bassour and Mount Wilson, though these latter places are separated in longitude by one-third of the earth's circumference. These measures have been supplemented by observations with a pyrheliometer in a manned balloon which reached the altitude of nearly 25,000 feet, where the pressure of the atmos phere was 298 mm. of mercury, and also• by pyrheliometer observations in a free balloon, which reached the great height of 72,000 feet at a pressure of 30 mm. of mercury. Observa tions along parallel lines have been made by Abbot for the purpose of investigating the ab sorption of the gases forming the solar en velope. The results from 1,000 determinations of the solar constant have proved conclusively that the so-called constant of solar radiation is 1.93 calories per square circumference per minute, that this value is not a constant, but that it varies as much as 10 per cent, that this variation is caused by changes in the absorption of the sun's atmosphere, and that as a result of these variations in the amount of heat sent out by the sun, there are parallel changes in terrestrial temperatures. The interdependence of these variations brings to view one of the most important developments of astrophysics. That terrestrial temperatures should be shown to be so closely connected with variations in the sun's heat is very remarkable; it would be very valuable if it were possible to forecast tem peratures on the earth as a result of solar ob servations—a result which is not impossible.