158 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



was through her starboard bow and port quarter; and it ap- 

 pears to be probable that this permanent magnetism now 

 continues to have its direction not very different from what 

 it was originally, suggesting a remarkable stability ; which 

 is not altogether improbable, in view of the admirable quali- 

 ty of the iron in the hull of the vessel. Report of the Secre- 

 tary of the JSfavy, 1873, p. 80. 



VARIATION IX MAGNETIC DECLINATION. 



Wolf, in the thirty-fifth number of his Astronomical Notices, 

 has endeavored to demonstrate that, at least for Europe, the 

 variation in magnetic declination at any place is very accu- 

 rately represented by a certain constant, plus as many minutes 

 of arc as are equal to the number found by taking four and a 

 half per cent, of the numbers published by him as represent- 

 ing the relative frequency of solar spots for any given year. 

 These constants are given by him for twenty-six places in 

 Europe and America. Thus, for instance, at Philadelphia the 

 variation of magnetic declination is represented by 6.88' + 4-|- 

 per cent, of Wolf's solar-spot-frequenej' number for any given 

 year. Astron. JSFach., LXXXIIL, 286. 



EFFECT OF FOG ON SOUND. 



Professor Reynolds, of Manchester, endeavors to explain 

 the fact that sound does not readily penetrate fog. He shows 

 that the particles of water do not, as it has sometimes been 

 supposed, break up the waves of sound by small reflections 

 in the same way as they scatter the waves of li^ht, but that 

 the destruction of sound is due to the fact that when fo<m*V 

 air is accelerated or retarded, the drops of water move 

 through the air, and expend energy in fluid friction. He 

 examines, further, the relation between the size of the drops 

 and their effects. He finds, in the first place, that if the air 

 is subjected to a uniform acceleration, then the energy dis- 

 sipated by the drops in a given time is proportional to the 

 square root of the diameter of the drops. Starting with drops 

 the size of rain, their effect will increase as their size dimin- 

 ishes, at first in the direct proportion, then more and more 

 slowly until a certain minuteness is reached, after which, as 

 the drops become still smaller, their effect will begin to di- 

 minish, at first slowly, but in an increasing ratio, tending to- 



