B. TERRESTRIAL PHYSICS AND METEOROLOGY. 99 



come to be generally understood, at least by those govern- 

 ment meteorologists who are engaged in weather predictions, 

 that the most important element of success in this branch of 

 meteorology is a better knowledge of the vapor of the at- 

 mosphere in all its various states and changes. This science 

 will, indeed, not make the advances it is designed to make 

 until meteorologists generally recognize the necessity of 

 equipping their first-class observatories with the requisite 

 appliances (both instruments and physicists) for carrying on 

 those physical researches which are intimately allied to their 

 own science. 12 A, IX., 164. 



THE TEMPERATURE OF ST. PETERSBURG. 



Rikatcheff has undertaken the laborious work of apply- 

 ing to the observations of temperature at St. Petersburg the 

 method of computation which enables the daily rate of change 

 of temperature to be represented by the simple harmonic 

 formula frequently known as Bessel's. To this end Rikatcheff 

 has employed the observations of twenty-one years, ending 

 in 1862, and made during the greater part of this period for 

 each hour of the day. He has studied the interesting ques- 

 tion of the rate of change of temperature in cloudy and in 

 clear weather a refinement that has, perhaps, never been in- 

 troduced on so extensive a scale as he has had the oppor- 

 tunity of doing. For St. Petersburg, he finds that there is 

 most cloudy weather in the month of December, and most 

 clear weather in July and August : a rule that Wild has 

 shown to be true for nearly the whole of Russia. The maxi- 

 mum cloudiness in November and December is about 72 per 

 cent. ; the minimum in June and July about 42. When the 

 sky is clear, the daily increase of temperature is a maximum 

 in May; while the nightly decrease is a maxim un> in Decem- 

 ber. The days when the daily increase and the nightly de- 

 crease are nothing are in March and September. When the 

 sky is covered with clouds, the value of the diurnal change 

 is found to vary regularly with the seasons, but depends, in 

 general, on rules such as the following : In spring, the tem- 

 perature does not increase, but rather diminishes; in win- 

 ter it increases. The greatest cooling occurs in the month 

 of June, and the greatest diurnal increase of temperature in 

 the month of December. These striking phenomena are ex- 



