THE AIR OF TOWNS. 371 



a very typical and dense Leeds fog. The more dust particles tliere are, 

 the thicker the fog. 



Before passing to the subject of town fog, I should like to say a word 

 or two about the weight and number of those dust particles which we 

 see play such an important part in the production of fog. The experi- 

 ments we have just seen have been turned to account by the distin- 

 guished physicist John Aitken, to determine the number of dust 

 particles in the air. By using a small vessel and dusty air largely 

 diluted with air free from dust, he has succeeded in producing an 

 apparatus, in which the dew drops or mist drops are sufficiently small 

 in number to be counted. As the apparatus is exceedingly simple 

 in construction, I propose to explain it. Fig. 26 represents the 

 instrument, which for the sake of explanation, is drawn partly in sec- 

 tion. It consists of a shallow circular metal box of known capacity, 

 furnished top and bottom with glass plates. It stands upon two cylin- 

 ders opening into it. One cylinder forms a small air pump and con- 

 tains a piston. The other is provided with three taps, the bores of 

 which hold a measured volume. The top tap holds the smallest and 

 the bottom one the largest volume. Below these there is a plug of cot- 

 ton wool, and at the bottom of cylinder, which is closed at the end, is 

 a small hole through which air can enter. Above the metal box is a 

 magnifying lens and below a reflector. The lower glass plate of the 

 box is divided into measured squares, etched on the glass. The atmos- 

 phere within the box is kept saturated with moisture by means of 

 strips of damp blotting paper. By drawing down the piston with the 

 taps in the position shown in the diagram, air enters the metal box 

 through the cotton ping, which frees it from dust. To test a sample of 

 air, one of the taps (determined by the amount of dust present) is 

 turned through a right angle so that the bore is horizontal. It now 

 communicates directly with the outside air as represented at a, which 

 shows it in section. By turning it back, the bore again communicates 

 with the metal box. The piston, which is at the top, is now drawn 

 down and the sample of dusty air is drawn up along with filtered air 

 into the metal box. By again raising the piston and drawing it down 

 rapidly, a deposition of moisture occurs, which falls in drops on the 

 glass squares, such as is represented on the top squares in the diagram 

 at I). These drops are counted and from this the number of dust parti- 

 cles may be ascertained. 



The following are the average number of dust particles in town and 

 country air taken from Aitken's observations : Country, 8,000 to 100,000 

 per cubic inch; town, 1,000,000 to 50,000,000 per cubic inch. I was not 

 satisfied with simply exhibiting Mr. Aitken's results, and so I borrowed 

 the instrument, which he kindly placed at my service, in order to find out 

 the character of the air in Leeds. The following results were obtained 

 on a fine day with the wind blowing from the northwest. The relative 

 position of the places of observation are noted on the diagram (fig. 27). 



