THE ATMOSPHERE AND FOG-SIGNALING. 549 



tion, the sound at two miles' distance on July 1st must have had more 

 than forty times the intensity which it possessed at the same distance 

 at 3 p. m. on the 3d. 



"On smooth water," says Sir John Herschel, "sound is propa- 

 gated with remarkable clearness and strength." Here was the con- 

 dition ; still, with the Foreland so close to us, the sea so smooth, and 

 the air so transparent, it was difficult to realize that the guns had been 

 fired, or the trumpets blown at all. What could be the reason ? Had 

 the sound been converted by internal friction into heat, or had it been 

 wasted in partial reflections at the limiting surfaces of non-homoge- 

 neous masses of air ? I ventured, two or three years ago, to say some- 

 thing regarding the function of the imagination in science, and, not- 

 withstanding the care then taken, to define and illustrate its real 

 province, some persons, among whom were one or two able men, 

 deemed me loose and illogical. They misunderstood me. The faculty 

 to which I referred was that power of visualizing processes in space, 

 and the relations of space itself, which must be possessed by all great 

 physicists and geometers. Looking, for example, at two pieces of pol- 

 ished steel, we have not a sense, or the rudiment of a sense, to distin- 

 guish the inner condition of the one from that of the other. And yet 

 they may differ materially, for one may be a magnet, the other not. 

 What enabled Ampere to surround the atoms of such a magnet with 

 channels in which electric currents ceaselessly run, and to deduce from 

 these pictured currents all the phenomena of ordinary magnetism? 

 What enabled Faraday to visualize his lines of force, and make his 

 mental picture a guide to discoveries which have rendered his name 

 immortal? Assuredly it was the disciplined imagination. Figure 

 the observers on the deck of the Irene, with the invisible air stretch- 

 ing between them and the South Foreland, knowing that it contained 

 something which stifled the sound, but not knowing what that some- 

 thing is. Their senses are not of the least use to them ; nor could all 

 the philosophical instruments in the world render them any assist- 

 ance. They could not, in fact, take a single step toward the solution 

 without the formation of a mental image ; in other words, without the 

 exercise of the imagination. 



Sulphur in homogeneous crystals is exceedingly transparent to 

 radiant heat, whereas the ordinary brimstone of commerce is highly 

 impervious to it the reason being that the brimstone does not possess 

 the molecular continuity of the crystal, but is a mere aggregate of 

 minute grains not in perfect optical contact with each other. Where 

 this is the case, a portion of the heat is always reflected on entering 

 and on quitting a grain ; hence, when the grains are minute and nu- 

 merous, this reflection is so often repeated that the heat is entirely 

 wasted before it can plunge to any depth into the substance. The 

 same remark applies to snow, foam, clouds, and common salt, indeed 

 to all transparent substances in powder ; they are all impervious to 



