Awhile back, an email correspondent with a Finnish email name,  SisuSuku,  
sent me the following, inquiring as to whether I though Twain  had 
plagiarized the passage. My response at the time was that he embellished a  good 
idea, using a concept borrowed from another. Not literal  plagiarism, and 
something often done by writers. It doesn't mean Twain had  not seen the effect; 
simply that the "balloon voyages" concept was not  original to him. I don't 
see it as plagiarism. But I would appreciate  Comment from others.   Bob 
Stewart

In the December, 1868,  Overland Monthly., Ben Avery wrote:
"[The water of Lake Tahoe] is wonderfully  transparent, and the sensation 
upon floating over and gazing into its still  bosom, where the gray granite 
bowlders can be seen far, far below, and large  trout dart swiftly, incapable 
of concealment, is almost akin to that one might  feel in a balloon above 
the earth. The color of the water changes with its  depth, from a light 
blueish green near the shore, to a darker green further out,  and finally to a 
blue so deep that artists hardly dare put it on canvas. . .  ."


In Roughing It (1872) Twain wrote:
"So singularly  clear was the water, that where it was only twenty or 
thirty feet deep the  bottom was so perfectly distinct that the boat seemed 
floating in the air! Yes,  where it was even eighty feet deep. Every little 
pebble was distinct, every  speckled trout, every hand's- breadth of sand. Often, 
as we lay on our faces, a  granite boulder, as large as a village church, 
would start out of the bottom  apparently, and seem climbing up rapidly to 
the surface, till presently it  threatened to touch our faces, and we could 
not resist the impulse to seize an  oar and avert the danger. But the boat 
would float on, and the boulder descend  again, and then we could see that when 
we had been exactly above it, it must  still have been twenty or thirty 
feet below the surface. Down through the  transparency of these great depths, 
the water was not merely transparent, but  dazzlingly, brilliantly so. All 
objects seen through it had a bright, strong  vividness, not only of outline, 
but of every minute detail, which they would not  have had when seen simply 
through the same depth of atmosphere. So empty and  airy did all spaces seem 
below us, and so strong was the sense of floating high  aloft in 
mid-nothingness, that we called these boat-excursions  'balloon-voyages.' "