192 CARNEGIE INSTITUTION OF WASHINGTON. 



tions are possible, can not be distinguished from the influence of an error 

 in the value of the solar parallax used in correcting for the relative velocity of 

 Venus and the earth. 



Differential measures at the limb and terminator can be used to determine 

 the rotation of the planet. Measures of this character give a small negative 

 value for the equatorial velocity of Venus, with a probable error of the same 

 order. It is here assumed that the plane of the planet's equator is in the 

 plane of its orbit. A similar investigation made some years ago by Slipher 

 at the Lowell Observatory also gave a small negative value for the equatorial 

 velocity with a relatively large probable error. It seems very improbable 

 that Venus rotates in a direction opposite to that of the earth and Mars, 

 and if we assume that the rotation is really direct and that the negative 

 values are due to errors of observation, we may interpret the result in the 

 light of its probable error by saying that the chances are about ten to one 

 that the period is longer than 20 days. 



It is possible that the equator of Venus is nearly perpendicular to the 

 plane of the planet's orbit. W. H. Pickering has interpreted certain of his 

 observations in this way and has given the position of the axis of rotation. 

 Spectrograms have been taken at favorable times and with orientations of 

 the slit suitable for testing this possibility, but with results which are de- 

 cidedly against the large inclination of the equatorial plane arrived at by 

 Pickering. 



RADIOMETRIC OBSERVATIONS OF THE MOON AND PLANETS. 



Pettit and Nicholson have studied the radiation from the moon. Mercury, 

 Jupiter, Saturn, Uranus, Neptune, and the asteroid Vesta with a vacuum 

 thermo-couple having a rock-salt window furnished by Dr. Abbot, of the 

 Smithsonian Astrophysical Observatory. Observations have also been 

 made through the water-cell, which transmits radiation only to 1.3 n, and 

 through the microscope cover-glass, which transmits to 7 /x in the infra-red. 

 By combining the results and correcting for reflection from the cells the 

 amount of energy which enters our atmosphere through Fowle's transmission 

 band 8 m to 14 /x can be measured. This quantity, which Pettit and Nichol- 

 son have called "planetary radiation," is the radiation emitted by the heated 

 surface of the planet, as distinguished from the general radiation which 

 includes reflected hght. 



The observations indicate that Jupiter and Saturn emit about 6 per cent 

 of planetary radiation as against 74 per cent from the moon. Drift curves 

 of the moon taken near the first or last quarter show that the total radiation 

 per unit of area of the illuminated portion is proportional to the distance 

 from the limb, being zero at the terminator. The transmission of the water- 

 cell is nearly constant over 70 per cent of the region from the limb to the 

 terminator, and then rises gradually to double its value at the limb. The 

 planetary radiation seems to be nearly the same over the dark areas of the 

 moon as over the brighter portions. In the case of Jupiter, the drift curves 

 show a falling off in total radiation near the limb, as well as in visual light. 



Observations of Mercury were made at western elongation on June 17 and 

 21. The planet was found to emit 74 per cent of planetary radiation, a value 

 identical with that obtained for the moon near the same phase. In addition, 

 the distribution of energy in the three regions, 0.3 m to 1.3 ai, 1.3 m to 5.5 //, 



