METEOROLOGY. 489 



" (2) Local or artificial impurities — dust, gases, or smoke from fac- 

 tories, burning forests, volcanic eruptions, the salts due to ocean spray, 

 dust due to roads, sandy deserts, &c. — can hardly be considered as be- 

 longing to general meteorology, but are important in local climatology. 



" (3) More general and more important are the minute particles of dust 

 that almost elude microscopic vision, and which are in reality spores 

 or germs of organic life; to these are due the various processes of fer- 

 mentation and putrefaction, and especially the large class of diseases 

 known as miasmatic, which were formerly attributed to noxious gases, 

 and to which all animal life is very susceptible. The warm, moist cli- 

 mates are in general most favorable to these germs; they are compar- 

 atively rare in very dry climates and over desert places; they are also 

 rare over the sea and during sea breezes on the sea coast, and also after 

 a rain has washed the atmosphere pure and brought down fresh air 

 from above. [The effect of rainfall in dragging down the air which 

 then flows outward from under the rain was fully recognized by Espy 

 and Henry in 1S1 0, and has frequently been dwelt upon by the present 

 writer, so that it can hardly be called a new observation, as is done by 

 Hann on page 50. Experiments for the determination of the amount 

 of air brought down were verbally suggested to Professor Pickering in 

 1871 as a subject for experimentation by his students in physics.] 



" (4) Either ozone or the hyper-oxide of hydrogen, or possibly nitric 

 oxide, is apparently at times efficacious in increasing the oxidizing 

 effect of the air, the result being that all organic matter is destroyed 

 thereby. The methods of observing these gases are, however, too un- 

 certain to allow of comparison between different localities, or even dif- 

 ferent portions of the same series. 



"(5) The electrical condition of the atmosphere is not known to exert 

 any considerable direct influence upon life, nor have we even observations 

 that would allow of comparisons of various localities in respect to the 

 electric potential. This subject is therefore to be omitted, at least at 

 present, from climatology, notwithstanding many popular expressions 

 of belief in its importance. The occurrence of thunder-storms has al- 

 ready been considered — see V (3), (g) — but this is because of other feat- 

 ures than the electric phenomena. 



"X. Phaenologic observations have frequently been introduced as a 

 guide to comparative climatology, but the fact that plants can accom- 

 modate themselves to climatic peculiarities renders this a study that must 

 be pursued very cautiously. As yet it has not been possible to state 

 the dependence of the development of plant life upon temperature so 

 securely as to allow of inversely inferring the temperature from the ob- 

 served stage of development. However, such studies are not to be 

 wholly discarded ; they are especially useful as indications of the cli- 

 matic differences at different elevations on the slope of a mountain; 

 also when a large number of special plants are studied they show the 

 retardation of vegetation in northern regions. Thus, for example, the 

 blossoming of plants in early spring is at Trieste, Gorz, and Villa Carlotta 



