Sound-Shadows in Water, 



105 



of the pile casting the shadow ; hence the shadow of the 

 cylindrical pile extended back for about 12 feet between sen- 

 sibly parallel vertical planes, and its boundaries, at this distance, 

 were still sharply defined. 



Fie. 3. 



^m 



It is evident that, if the explosive centre were of insensible 

 magnitude, the horizontal thickness of the geometrical shadow 

 of the pile, at a distance of 12 feet beyond it, would be aug- 

 mented in the ratio of 40 to 40 +12,' or of 40 to 52, these 

 nunibersbeing the distances in feet from the centre: so that, 

 if the thickness of the shadow at the pile were 12 inches, its 

 thickness at 12 feet beyond would be 15'6 inches. If, how- 

 ever, the explosive energy occupied more or less space (as was 

 the casein relation to the " giant-powder " cartridges), the 

 thickness of the geometrical shadow or umbra cast bv the 

 pile might not increase sensibly with augmenting distance ; 

 and, indeed, in case the exploding body exceeded 12 inches in 

 diameter the thickness of the shadow would diminish with 

 increasing distance from the obstructing pile, as in the case 

 of the umbra cast by an opaque body which is smaller than 

 the luminous source. 



14. Another Phenomenon observed. — Another interesting 

 phenomenon came under our notice during the execution of 

 these experiments. It was the singular effects observed on 

 the surface of the water (when perfectly calm and glassy) for 

 ascertain area around the point immediately over "the explo- 

 ding cartridge. Simultaneous with the first shock (11) trans- 



