NO. 7 THE PYRANOMETER ABBOT AND ALDRICH 9 



auxiliary apparatus used with the Ang-strom pyrheHometer. Its read- 

 ings on the sky and sun by day appear to be truly expressed in calories 

 per square centimeter per minute, for in solar comparisons values 

 found agree within experimental error for all zenith distances with 

 those of our standardized pyrheliometers. We are undertaking fur- 

 ther experiments to test its accuracy for long wave-rays such as 

 compose nocturnal radiation. While we have hitherto employed only 

 ultra-violet crown glass screens, it is obvious that such screens might 

 be covered with stained gelatine, or other screens of special glass 

 employed to restrict the measurements to special regions of spectrum, 

 as might be desirable in botanical investigations. While the two strip 

 form is preferable from its greater freedom from temperature dis- 

 turbances, the single strip form is so much more sensitive that for 

 observations in deep shade, as in a forest, it would be more suitable. 



As in the case of the silver disk pyrheHometer, we are authorized 

 to state that the Smithsonian Institution will undertake to furnish 

 pyranometers at cost to those who are engaged in investigations which 

 will be greatly promoted by the use of this instrument. The cost 

 cannot yet be exactly estimated, but it will not exceed $150 for the 

 pyranometer itself. This does not, of course, include a galvanome- 

 ter, ammeter, or batteries. Suitable slide wire resistances will be 

 included. If desired, an equatorial mounting additionally will be 

 be furnished at cost, so that the instrument can be used as a pyr- 

 heHometer at right angles to the solar beam. 



SUMMARY 



The authors have designed and tested with satisfactory results 

 an instrument for measuring solar and sky radiation by day and 

 terrestrial radiation by night. Two forms of the instrument are 

 described. Either form will be furnished at cost by the Smith- 

 .sonian Institution to institutions or individuals doing important 

 investigation which will be promoted by using the instrument. 



