238 SECTION MECHANICS. 



passing through it, or of proof spirit, according to the way in which the 

 instrument is contrived and set. I am assured it works with great 

 accuracy, and being put under a glass case and locked up, the Russian 

 excisemen have not half the trouble with the distillery that our ex- 

 cisemen have. The distiller cannot tamper with it in any way without 

 being found out. The pipes coming from the still are completely in 

 sight ; he cannot tap them without the hole being discovered, and he 

 cannot tamper with this machine without showing signs of it. I think 

 I have now said all I need say with regard to these measurements, and 

 I hope I have made myself clear to you. 



The PRESIDENT: I have only now to call upon you to pass a vote 

 of thanks to Mr. Merrifield for his very able and lucid explanation of 

 the different modes of measuring solids. He undertook this task only 

 upon two days' notice, and is therefore entitled to our special thanks. 



The vote of thanks having been passed unanimously, 



The PRESIDENT: I will now call on Sir Wm. Thomson to give us 

 some explanation upon that most difficult and important subject of 

 electrical measurements. I am afraid Sir William will not be able to 

 go as fully into the subject as it deserves to be gone into, inasmuch as 

 our time is exceedingly limited, but I am sure that whatever falls 

 from Sir Wm. Thomson will convey information to all, and that we 

 shall profit by it. 



Professor Sir W. THOMSON, L.L.D., F.R.S. : The beginning of elec- 

 trical measurements, are, I believe, the measurements of Robinson in 

 Edinburgh, and of Coulomb in Paris of electrostatic forces. The great 

 results which followed from those measurements illustrated how 

 important is accurate measurement in promoting thorough scientific 

 knowledge in any branch of physical science. The earlier electricians 

 merely describe phenomena attractions and repulsions and flashes 

 and sparks, and the nearest approach to measurement which they 

 gave us, was the length of the spark under certain circumstances, 

 the other circumstances on which the length of the spark might 

 depend being left unmeasured. By Robinson's and Coulomb's 

 experiments was established the law of electrostatic force, according 

 to which two small bodies, each electrified with a constant quantity 

 of electricity, exercise a mutual force of attraction or repulsion, accord- 

 ing as the electricity is similar or dissimilar, and which varies 



