94 ESSAYS BIOGRAPHICAL AND CHEMICAL 



who made that noise : be so good as to leave the room/ 

 said Sir William. Mr. Smith blushed and retired. Then 

 came the explanation. Mr. Macfarlane had gone below 

 the sloping tier of seats; had accurately diagnosed the 

 precise position of Mr. Smith's erring foot, and had 

 accurately measured the distance from the two walls. 

 These measurements were reproduced in full view of the 

 students, and the advantages of the system of Cartesian 

 co-ordinates were experimentally demonstrated, while 

 justice was satisfied. 



Owing to an accident, Sir William was lame ; but it did 

 not interfere with his activity of body. Indeed, it lent 

 emphasis to his amusing class demonstration of ' uniform 

 velocity,' when he inarched backwards and forwards 

 behind his lecture-bench, with as even a movement as his 

 lameness would permit ; and the class generally burst into 

 enthusiastic applause when he altered his pace, and intro- 

 duced us to the meaning of the word ' acceleration.' 



In his laboratory Sir William was a most stimulating 

 teacher, though his methods were not those which have 

 since been introduced into physical laboratories. I re- 

 member that my first exercise, which occupied over a 

 week, was to take the kinks out of a bundle of copper 

 wire. Having achieved this with some success, I was 

 placed opposite a quadrant electrometer and made to 

 study its construction and use. I was made to determine 

 the potential difference between all kinds of materials, 

 charged and uncharged ; and among others between the 

 external and internal coatings of a child's balloon, black- 

 leaded externally and internally, and filled with hydrogen. 

 Nor was the Professor always prescient. On one occasion 

 I turned the handle of a large electrical machine, while 

 he held a two-gallon Leiden jar by its knob, and charged 

 the outside coating. It was not until it was fully charged 

 that it occurred to one of us that while the jar was quite 



