1874] WRITINGS OF JOSEPH HENRY. 373 



probably due to the reflection and radiation of the heat from 

 the land, which converted the watery particles into invisi- 

 ble 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 precipi- 

 tated in rain." This rising of fog is not due, according to 

 the doctor, to a surface-wind from the west pressing 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 current, 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 

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

 distance of objects as seen in a fog is also a remarkable phe- 

 nomenon when observed for the first time. A piece of float- 

 ing wood at a little distance is magnified into a large object, 

 and after much experience the doctor was not able to over- 

 come 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 impres- 

 sion of the size and distance of the objects 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 distance 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, particularl}' during the 

 summer in clear nights, with wind probably from a north- 

 erly 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 



