METEOROLOGY AND ALLIED SUBJECTS. 271 



Professor Wild. He also shows that both observations and theory con- 

 cur in proving that the ordinary law 



log A ^> = A — B X p 



(where A p expresses amplitude of any periodical oscillation at the 

 depth p under the surface and A and B are two constants) has no appli- 

 cation to the upper surface of the earth or to the daily oscillations, but 

 is an approximate altitude formula only for greater depths. He gives the 

 most accurate combinations of hours for observations of temperature at 

 the earth's surface and the corrections for the ordinary hours of observa- 

 tion, 7 a. ra. and 2 and 9 p. m. After a thorough discussion of the obser- 

 vations made elsewhere through the globe, Wild shows the incorrectness 

 of Boussingaulfs conclusion that in the neighborhood of the Equator, 

 under a protecting roof, the annual and the daily variations in temper- 

 ature within the earth disappear at a depth of less than 0.5 meter. 



Even with an annual change in temperature of only l^.o C, and 

 with the largest value of B at Nukuss, the depth p at which the oscil- 

 lation of temperature is only 0.1 of a degree is about five meters; at 

 Trevandrum the depth is 9.G meters. 



In general we may say that the stratum of earth whose temperature 

 sensibly vtiries in the course of the year is in various places rarely less 

 than G meters and rarely greater than 33 meters. In general. Wild 

 concludes that the condition of the outer surface of the earth with re- 

 spect to temperature can be well presented by means of the well-known 

 sine formula of Bessel. If, now, the periodical changes in the tempera- 

 ture at the surface jienetrate to the lower depths, then, according to the 

 theory of Poisson, there should be a gradual diminution in the ampli- 

 tude and change of phase of the movement of all the periodical terms; 

 but the attempt to represent observations by the strict theory of Pois- 

 son is so unsatisfactory as to render the theory but of little use when 

 it is applied to changes of short period, such as a day or a year, while 

 the longer periods such as 13 or 18 years are fairly represented. 



The second thermal constant of the earth, namely, its conductibility, 

 can at present not be satisfactorily determined, but the constant ratio 

 between conductibility and capacity for heat is determined with con- 

 siderable accuracy. {Z. 0. G. ill., XIV, p. 272.) 



Pernter gives a comparison of the methods of measuring the chem- 

 ical intensity of the light. These methods, so far as they have j'et been 

 proposed, are as follows: 



First. Bunsen and Eoscoe's chlorine and hydrogen photometer. 



Second. Bunsen and Roscoe's photographic actinometer, wliich meas- 

 ures the chemical intensity by the blackening of chloride of silver paper. 



Third. Marchand's photantitypimeter. (This awkward name is given 

 by Marchand himself, based on the Greek a>rtru-:a.) The principle of 

 his apparatus consists in the determination of the quantity of carbonic 

 acid eliminated from a mixture of chloride of iron and oxalic acid 

 exposed to the influence of light. 



