RESEARCHES IN SOUND. 459 



current, that in sailing in some cases only a few lengths of a ship the 

 temperature of the water will change from 70"^ to 50°. The fog fre- 

 quently comes rolling in with the speed of a race-horse; in some cases 

 while dredging, hapj^ening to turn my eyes to the south, a bank of fog 

 has been seen approaching with such rapidity that there was scarcely 

 time in whicli to take comi)ass-bearing of some object on shore by which 

 to steer, before I would be entirely shut in, perhaps for days together." 

 lie also mentions the fact that it frequently happened during a warm 

 day, while a dense fog existed some distance from the shore, close in to 

 the latter there would be a space entirely clear; this was probably due 

 to the reflection and radiation of the heat from the land, which converted 

 the watery particles into invisible vapor. 



Dr. Stimpson has also noticed another phenomenon of some interest. 

 "When a dense fog, coming in regularly from the sea, reaches the land, 

 it gradually rises in the atmosphere and forms a heavy, dark cloud, which 

 is frequently precipitated in rain." This rising of fog is not due, accord- 

 ing to the doctor, to a surface-wind from the west i^ressing under it and 

 buoying it upward, since the wind at the time is from the ocean. It is 

 probably due to the greater heat of the land causing an upward cur- 

 rent, which, when once started, by its inertia carries the cloud up to a 

 region of lower temperature, and hence the precipitation. The height 

 of the fog along the coast is not usually very great, and can be frequently 

 pverlooked from the mast-head. The deception as to size and distance 

 of objects as seen in a fog is also a remarkable phenomenon when ob- 

 served for the first time. A piece of floating wood at a little distance is 

 magnified into a large object, and after much experience the doctor was 

 not able to overcome the delusion. It is said that the sailors in the Bay 

 of Fundy prefer of two evils a fog that remains constant in density to 

 one that is variable, although the variation may be toward a greater 

 degree of lightness, on account of the varying intensity producing a 

 varied and erroneous impression of the size and distance of the object 

 seen through it. It is also his impression that sound can be heard as 

 well during fog as in clear weather, although there is a delusion even 

 in this, since the source of sound, when seen, appears at a greater dis- 

 tance than in a clear atmosphere, and hence the sound itself would 

 appear to be magnified. 



Fogs also exist on the Mississippi, especially on the lower portion of 

 the river. They are of two classes, those which result from the cooling 

 of the earth, particularly during the summer in clear nights, with wind 

 probably from a northerly direction, followed by a gentle, warm wind 

 from the south surcharged with moisture, and the other induced by the 

 water of the river, which, coming from melting snow of northern regions, 

 is colder than the air in the vicinity. The air over the river being thus 

 cooled below the temperature of a gentle wind from the south, the 

 moisture of the latter is precipitated. This fog, which occurs in the last 

 of winter, during the spring, and beginning of summer, is very dense, 



