214 SECTION MECHANICS. 



science ; and the measurement of the intensity of light by photo- 

 metry, which, while involving also physical problems of the highest 

 order, has an important bearing also upon applied science. The 

 principal methods that have been hitherto employed in photometry 

 are by the comparison of shadows, that of Rumford and Bouguer ; by 

 employing a screen of paper with a grease-spot, the lights to be com- 

 pared being so adjusted that the spot does not differ in appearance 

 from the rest of the paper, Bunsen's method ; Elster's, by determining 

 in combustion the amount of carbon contained in a given volume of a 

 gas ; and the one lately introduced by Professor Adams and Dr. 

 Werner Siemens, by measuring the variation in the electrical resistance 

 of selenium, under varying intensities of light. 



Before concluding, I wish to call your attention to two measuring instru- 

 ments which do not fall within the range of any of the divisions before 

 indicated. The first is an apparatus designed chiefly by my brother, 

 Dr. Werner Siemens, by which a stream composed of alcohol and 

 water, mixed in any proportion, is measured in such a manner that 

 one train of counter wheels records the volume of the mixed liquid ; 

 whilst a second counter gives a true record of the amount of absolute 

 alcohol contained in it. The principle upon which this measuring 

 apparatus acts may be shortly described thus : The volume of liquid 

 is passed through a revolving drum, divided into three compartments 

 by radial divisions, and not dissimilar in appearance to an ordinary 

 wet gas-meter ; the revolutions of this drum produce the record of 

 the total volume of passing liquid. The liquid on its way to the 

 measuring drum passes through a receiver containing a float of thin 

 metal filled with proof spirit, which float is partially supported by 

 means of a carefully-adjusted spring, and its position determines that 

 of a lever, the angular position of which causes the alcohol counter to 

 rotate more or less for every revolution of the measuring drum. Thus, 

 if water only passes through the apparatus, the lever in question 

 stands at its lowest position, when the rotative motion of the drum 

 will not be communicated to the alcohol counter, but in proportion as 

 the lever ascends a greater proportion of the motion of the drum will 

 be communicated to the alcohol counter, and this motion is rendered 

 strictly proportionate to the alcohol contained in the liquid, allowance 

 being made in the instrument for the change of volume due to 



