486 SCIENTIFIC RECORD FOR 1833 



' actinic rays ; ' they have variations of wave length and intensity, i. e. T 

 rapidity and extent of vibrations, and one and the same ray will produce 

 either or all of the above three effects (heat, light, chemism), according 

 to the nature of the surface or body on which it falls. The energy or 

 vis viva of a ray of a given wave length and intensity is proportional 

 to the quotient of the square of the amplitude divided by the square of 

 the wave length; it is the total radiant energy that is desired in climatic 

 researches. The most recent observations indicate that the central part 

 of the solar spectrum is most energetic not only in producing light, but 

 also in respect to heat and chemical effects." 



Meteorology still has to deplcre the absence of sufficient observations of 

 the optical, thermal, and chemical effects of solar radiation. The im- 

 portance of insolation (the amount of direct sunshine) has long been rec- 

 ognized by physicians, and by agriculturists, by its effect on animal and 

 vegetable life. Of the instruments with which regular observations have 

 been made in the effort to obtain comparative data, Hann mentions the 

 "black -bulb thermometer in vacuo," as used by the English as of the 

 first importance, andputsin the secondranktheArago-Davy actinometer. 

 [We may be allowed to add that this can only be allowable by reason of 

 the fact that no satisfactory study of these instruments has yet been pub- 

 lished, and our author's decision will perhaps be reversed after the pub- 

 lication of an exhaustive memoir by Ferrel, now in print.] 



"A record of phaenological phenomena, such as times of blossoming, 

 fruiting, &c, gives some interesting results bearing on the insolation. 

 The amount of heat reflected from the soil and water into the atmos- 

 phere has been approximately measured by Frankland, Dufour, &c, 

 and may amount to as much as 70 per cent. 



" III. The nocturnal cooling of the free surface of bodies due to radi- 

 ation of heat is another important subject that as yet eludes direct 

 observation. An approximation is sought by observing minimum radia- 

 tion thermometers fully exposed to the sky, and whose bulbs are respect- 

 ively on the ground and directly above a close-cropped grass lawn. 



" IV. The temperatures of the ground at the surface, and to the depth 

 of one or two meters, constitute a most desirable climatological element. 



" V. The measurement of atmospheric moisture, both vaporous and 

 condensed, ranks next to temperature in importance. The following 

 items are desirable : 



*" (1) Monthly means of the absolute, and 



" (2) Of the relative aqueous contents of the atmosphere. The absolute 

 measures are usually given in millimeters of vapor tension, but pref- 

 erably in weight of vapor per cubic meter. The relative humidity is 

 given in x^er cent. The ordinary observations with hygrometric apparatus 

 give the aqueous vapor in the immediate neighborhood of the observer, 

 and tell nothing as to the amount or tension at any considerable distance ; 

 the relative humidity, with the temperature of the air and the wind, gives 

 the evaporating power of the atmosphere, which has so great an influence 

 on plants and animals. 



