273 



climates upon our crops. Those tables must include at least the 

 following data : 



1. The mean temperature of the air in the shade. This may be 

 deduced most simply from the average of the daily maximum and 

 minimum temperatures. 



2. The mean temperature of a thermometer, preferably a black 

 bulb, but not in vacuo, exposed to the full sunshine and wind and 

 placed amid the foliage of the trees or the blades of the grain that 

 is to be studied, so that its temperature may be ap])roximately that 

 of the plant. This should also preferably be obtained by using 

 maximum and minimum thermometers. 



3. The temperature of the soil at depths of 1 inch and 6 inches, 

 corresponding to the depths of the roots of the plants. 



4. The hygrometric condition of the free air, which may be 

 expressed either as relative humidity or as dew point or as vapor 

 tension. The latter Avill be most convenient in all our calculations. 



5. The velocit}^ of the w^ind or its daily movement. 



6. The cloudiness of the sky. This may be obtained from the 

 ordinary estimates of cloudiness if these are made very frequently, 

 but with more ease and accuracy from some form of sunshine recorder. 



7. The total effective radiation from sun and sky. This may be 

 obtained from frequent observations of the Marie-Davy actinometer 

 or the so-called Arago-Davy conjugate thermometer, or Violle's conju- 

 gate bulbs, but still better when these are made self-recording, and 

 better yet from such forms of apparatus as the photantitupimeter or 

 phantupimeter of Marchand, or the radiometer of Bellani, which 

 Marie-Dav}^ has improved uj^on in the form described by him as the 

 vaporization lucimeter. (See Annuaire de Montsouris, 1888, p. 207, 

 or 1890, p. 61.) The methods of using these instruments are doubt- 

 less subject to improvement, but these or some more delicate sub- 

 stitutes are absolutely necessary in order to enable us to appreciate 

 the w^ork done by solar radiation. In the absence of instruments 

 we may use the maximum sunshine as diminished by the estimated 

 cloudiness. 



8. The actual evaporation from plants and soils, or in lieu of this 

 the evaporation recorded by the Piche or even older forms of evapo- 

 rimeters whose records are doubtless closely parallel to those of the 

 plants in the soil, but usually largely in excess of these. 



9. The total rainfall as measured by the ordinary rain gauges in 

 the experimental field. 



As an illustration of the convenience of such tables I have com- 

 piled the following table for Montsouris by pentads in so far as the 

 data is given by pentads by Descroix in the Annuaire for 1890. 

 Some of the data is obtained by interpolation from monthly values 



2667—05 M 18 



