January 3, 1913] 



SCIENCE 



29 



at 10 o 'clock, roughly, forenoon and evening, the 

 minima at 4 o 'clock, roughly, afternoon and 

 morning. 



All that is needed, apparently, to give the semi- 

 diurnal pressure curve, is a pressure impulse of 

 the same period, 12 hours, as that of the free 

 vibration of the atmosphere as a whole. And this, 

 it seems, is furnished by a forced forenoon baro- 

 metric maximum, due to the mterference of ver- 

 tical convection with the free flow of the air, fol- 

 lowed, six hours later, at the same place by a 

 forced afternoon barometric minimum, caused by 

 expansion and overflow. In other words, taken 

 together the forenoon and afternoon forced dis- 

 turbances appear to occur with the proper time 

 interval necessary to set up and maintain the 12- 

 hour free vibrations of the atmosphere. 



The course of events at each locality, affecting 

 the height of the barometer, appears to be sub- 

 stantially as follows : ( 1 ) A forced forenoon com- 

 pression of the atmosphere followed by its equally 

 forced afternoon expansion; both due to a single 

 heating and the two together forming one com- 

 plete barometric wave, with a lO-o 'clock maximum 

 and a 4-o 'clock minimum, in harmony with the 

 free vibration of the entire atmosphere shell. 

 (2) Non-disturbance through the night, or during 

 the period of a single free vibration. (3) Eepeti- 

 tion the following day of the forced disturbances 

 in synchronism with, and therefore at such time 

 as to reinforce, the free vibrations. The series of 

 disturbances of course is indefinitely great, alter- 

 nately forced and alternately free, but the result- 

 ing amplitudes of the barometric changes are 

 limited, through friction and through the absence 

 of perfect synchronism, to comparatively small 

 values. 



A Screen for Equalinng Star Magnitudes for 

 Transit Circle Worh: F. B. Littell. 

 The equalizing screen consists of two sets of 

 thin brass slats at right angles to each other, 

 intermeshed to form a rectangular, honeycomb 

 structure. The slats are 1/2 inch wide, 1/120 inch 

 thick and 1/5 inch apart, and make an angle of 

 45° to the plane of the meridian. Tljj whole is 

 mounted about 3 inches in front of the objective 

 and may be tilted about an axis parallel to the 

 horizon, the operation being efl'ected and the 

 amount read at the eye end. The light which falls 

 on the objective is evenly distributed in nearly 

 rhombic areas. A tUt of 30° gives total extinc- 

 tion. The bright spectral images formed by the 

 bright stars which vary in character with the set- 



ting of the screen can be rendered practically 

 iavisible in a bright field by preliminarily re- 

 ducing the light by means of one or both of the 

 two wire mesh screens which have been used on 

 the instrument for several years and which are 

 retained for this purpose. 



24-inch Objective of the Sproul Telescope: J. A. 



Miller and R. W. Mariott. 



The 24-ineh objective for the Sproul telescope 

 by the John A. Brashear Co. was completed and 

 mounted in the Sproul Observatory of Swarthmore 

 College in December of 1911. This paper dis- 

 cussed the quality of this objective as determined 

 by the Hartmann method. In February of 1912, 

 extra focal exposures on Capella were made 

 through a screen containing 44 circular holes, 

 33 mm. in diameter, so arranged as to cover 9 

 zones of the lens. The exposures were made on 

 Cramer Instantaneous Isochromatic plates. A yel- 

 low ray filter made by Wallace in accordance with 

 the color curve of the objective was placed in front 

 of the plate and almost in contact with it. Later 

 a set of extra focal exposures were made on 

 Arcturus in the same way. In the meantime, how- 

 ever, a spring holding the lens in its place in the 

 cell was slightly loosened. The exposures on 

 Arcturus were through a screen containing 78 

 circular holes of 25 mm. diameter, so arranged 

 that it covered 10 zones. The measure demon- 

 strated that qualitatively the two tests showed 

 slight axial stigmatism, at approximately the same 

 points in the lens. Hartmann 's characteristic T 

 for the Arcturus test is 0.270 and for the Capella 

 test, 0.274, showing that the lens is an excellent 

 one. 

 The Orbits of 44 Eclipsing Binaries: Harlow 



Shaplet. 



Using the methods recently developed by Pro- 

 fessor Eussell, orbits have been computed for all 

 eclipsing variable stars for which there exists 

 reliable photometric data. Included in the ma- 

 terial discussed are unpublished observations made 

 by the writer with the Princeton polarizing pho- 

 tometer of the stars EZ Draconis, BX Draconis, 

 BB Draconis, WZ Cygni, AE Cygni, ZZ Cygni, 

 EW Capricorni, BX Herculis and BW Monocerotis. 

 The periods of a few stars have been redeter- 

 mined. Secondary minima have been found from 

 an analysis of maximum light observations in the 

 cases of U Cephei, B Canis Majoris, BZ Draconis, 

 ST Carinse, SU Centauri and SZ Centauri, and 

 possibly for S Cancri, 5 Librse and BW Mono- 

 cerotis. The investigation of the light curves of 



