180 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



reflector of Sir William Herschel, and which is at least of 

 the twentieth order of magnitude according to the scale used 

 by this astronomer, belongs in fact to the three hundred 

 and twentieth order of magnitude on the photometric scale. 

 The geometric scale offers none of these inconveniences, al- 

 though, on the other hand, it leaves something arbitrary in 

 the choice of the constant factor of the progression. In both 

 scales the standard of magnitude must be adopted as the fix- 

 ed point of departure ; this is an arbitrary point, whose se- 

 lection demands much careful consideration. The choice of 

 this unit of brightness may depend upon the following con- 

 siderations : First, it may be a star of invariable brightness 

 (if such exist). Second, it might be an artificial light, if we 

 take means at hand for producing a light of constant value. 

 Third, it may be determined by the effect upon either the 

 eye itself, or upon the inert bodies that are employed in the 

 photographic process. As regards the eye, it should be re- 

 membered that the image found upon the retina depends 

 upon the more or less perfect adjustment of the eye of the 

 observer, and, second, that the aperture of the pupil is va- 

 riable within very considerable limits. These two latter 

 sources of uncertainty may be remedied by simple means, 

 when it will be found that it is highly convenient to adopt 

 as a standard the faintest stars visible to the normal eye. 

 This unit having been determined by many observers for 

 many stars, the average of all will be a unit representing the 

 average sensitiveness of the human eye, and independent of 

 fluctuations in the brightness of the stars, and which there- 

 fore is sensibly constant. Bibliotli&que Unwerselle^ 1874, 209. 



FLOW OF AIR THROUGH ORIFICES. 



An extensive series of observations has been made upon 

 the flow of air through orifices, and its discharge under 

 great pressures, by Professor Fliegner and Dr. Zeuner, of 

 the Polytechnic School at Zurich. The velocity of dis- 

 charge can be obtained theoretically from the kinetic the- 

 ory of the constitution of gases, according to which theory 

 l the molecules are, at relatively great distances from each 

 other, moving in straight lines, except when they impinge 

 on each other, or on the walls of the contained vessel, in 

 which cases they rebound as if perfectly elastic. Applying 



