B. TERRESTRIAL PHYSICS AND METEOROLOGY. 99 



recommended by Lamont. It consists principally of a rect- 

 angular tube buried permanently in the earth, within which 

 five rectangular prisms of wood are placed, one above the 

 other, at different depths in the ground, and which, by a sim- 

 ple arrangement, can be easily and quickly drawn up. Each 

 of these tubes contains a thermometer, and there is a hole in 

 the side of the main tube, opposite to the bulb of the ther- 

 mometer, where the wood-work is cut away, and the opening 

 closed by a plate of thin sheet copper, whose temperature 

 may be presumed to be the same as that of the adjacent 

 ground. The depths at which the thermometers' bulbs re- 

 main are 4, 8, 12, 16, and 20 feet. Schenzl, as the result of 

 observations made during eight years, finds that the time 

 required for heat to penetrate to a depth of one meter is, on 

 the average, 21 days. 



THE INTENSITY OF TWILIGHT. 



Mr. Williams, a student of the Massachusetts Institute of 

 Technology, has made some observations to determine the 

 quantity of light reflected during the hours of twilight, when 

 the sun is at different distances below the horizon. The pho- 

 tometer employed was a slight modification of the Bunsen 

 photometer. The illuminated disk was exposed on one side 

 to the light from a standard candle, and on the other side to 

 the light admitted from the sky. Proceedings Am. Acad. 

 Arts and Sciences, 1875, 421. 



ON THE QUANTITY OF LIGHT REFLECTED BY THE SKY IN THE 



DAYTIME. 



The quantity of light reflected by the sky at any given 

 distance from the sun has long been a subject of meteoro- 

 logical observation, the first rude attempt at its measurement 

 being made by the use of Saussure's cyanoraeter. Some elab- 

 orate investigations liave been pursued for a long time at 

 the ^Eont-souris Observatory^ An interesting^ investigration 

 of the question by a simple photometric apparatus has re- 

 cently been published by Crosby as a student under Profess- 

 or Pickering of the Institute of Technology at Boston. His 

 objects were to determine, first, the absolute amount of light 

 received from the sky at different distances from tlie sun, and 

 second, to ascertain the law of diminution of lii^ht with in- 



