WEATHER BUREAU. 61 



ments owned and operated by the bureau itself, one at Washing- 

 ton, D. C, the other Northfiekl, Vt., and partly through the coopera- 

 tion of 18 additional stations distributed from Panama to Alaska 

 and from the Hawaiian Islands to Porto Kico. 



During the calendar year 1916, 131 earthquakes were felt within 

 the borders of the United States proper. The great majority of 

 these produced no damage whatever, and only six or seven were 

 severe enough to produce even slight damage. 



SOLAR RADIATION INVESTIGATIONS. 



Solar radiation measurements of much the same character as those 

 for 1915-16 have been obtained throughout the year at Washington, 

 D. C, Madison, Wis., Lincoln, Nebr., and Santa Fe, N. Mex., and 

 the results have been published each month in the Monthly Weather 

 Review. The instrumental equipment at Washington has been in- 

 creased by the purchase from the Smithsonian Institution of a 

 pyranometer, which will be used principally in restandardizing re- 

 cording pyrheliometers. 



Excellent observations were obtained of a cloud layer of high 

 haze that overspread the United States from the Atlantic coast south- 

 westward to southern California at the end of July, 1916. These in- 

 cluded measurements of the height of the haze layer, and its direc- 

 tion and velocity of movement, and descriptions of the brilliant twi- 

 light colors, especially the purple afterglows, that accompanied it. 

 The measurements of height and movement are in accord with similar 

 measurements of the movement of balloons above a height of about 

 16 kilometers, and are confirmatory of the existence of air currents 

 from the east at these high levels. In California and Arizona the 

 haze and the brilliant afterglows were observed until after the end 

 of 1916. 



Photometric measurements of the intensity of twilight previously 

 made at Mount Weather, Va., have been supplemented by further 

 measurements at Salt Lake City, Utah. These are summarized in the 

 Eeview for November, 1916. In connection therewith are published 

 tables showing the duration of both civil and astronomical twilight 

 at different latitudes; and the term "civil twilight," which does not 

 appear in English dictionaries, is definitely defined. 



At Salt Lake City photometric measurements have also been made 

 of the intensity of twilight illumination on a cloudless day with a 

 clear sky and on a similar day except for the presence of a dense 

 layer of surface smoke. The measurements include illumination 

 from direct sunlight, from diffuse skylight, and from the two com- 

 bined, which latter is the total daylight illumination. They show 

 that the total illumination averages abou.t one-third less on a smoky 

 day than on a clear day and that the illumination from direct sun- 

 light averages one-half less. 



A study has been made of the shading effect of Avire insect cages, 

 such as are employed by the Bureau of Entomology to protect plants 

 from insect pests, and also of various kinds of shade cloth employed 

 by tobacco growers in certain sections to improve the quality of the 

 tobacco leaf. The shading may be expressed by a simple mathemati- 

 cal formula, as has been shown in the Review for September, 1916. 



