452 POPULAR LECTURES AND ADDRESSES. 



accident, but with almost any other person would 

 have led to the instrument being destroyed by 

 the sulphuric acid getting shaken down into the 

 instrument below. A more convenient arrange- 

 ment of the pumice is now made, but that is 

 the only alteration besides the mechanical 

 arrangement of the disc which is better in the 

 portable electrometer as it now exists. Two of 

 these instruments have been sent out with the 

 Arctic expedition (of 1875-76). 



Just one word of practical advice with respect 

 to the electrometers. I have been continually 

 asked how to keep them in order, and have fre- 

 quently heard complaints that these will not hold ; 

 that they do not retain their charge. In each of 

 these electrometers there is a glass Leyden jar, 

 the heterostatic system being adopted in each of 

 them. It is necessary the insulation should be very 

 perfect, and then it all depends afterwards on the 

 cleanness and dryness of the surface of the glass. 

 If you will allow me to use the definition of Lord 

 Palmerston, when he said that, " Dirt is matter in 

 its wrong place," and to consider that water, or 



