210 ANNUAL OF SCIENTIFIC DISCOVERY. 



portion of the water into steam of excessive tension ; a tension such as 

 nothing can withstand. The terrific consequences are too often wit- 

 nessed in those fatal catastrophes, which have given to our western 

 rivers such a tragic reputation. 



No one can examine a list of western steam-boat excursions with- 

 out being forcibly impressed with the frequency of these accidents just 

 as the boat is starting from the wharf after landing. It seems to me 

 beyond doubt that many of these occur just in the manner now stated, 

 and from the deficiency of air-bubbles in the boiler. We see in this 

 reasoning, too, a sufficient explanation of dry steam ; or steam hotter 

 than its tension indicates. The heating is then going on faster than 

 the evaporation, and the steam is thus heated as if it were not in con- 

 tact with the water, or were in a vessel by itself. It is not always 

 that the remedy for a danger is as obvious and easily applied as in this 

 case. It is only necessary to keep the pump in steady, slow operation 

 while the engine is at rest. It should always be capable of an inde- 

 pendent movement, and should constantly, while a boat is going, be 

 kept at work, however slowly. By this means air for ebullition will al- 

 ways be supplied, and the accumulation of heat in a sluggish mass of 

 water cannot then go on until the explosive point is reached. 



The field over which I have thus rapidly traversed, is one requiring 

 much practical study for its full development and illustration. I could 

 not here give all which belongs to it without exceeding reasonable limits. 

 Nearly all the views which I have presented, were the result of my 

 own studies, so far as concerned my original acquaintance with them. 

 But I was happy to find that Denny and Henry had, in some points, 

 reached the same conclusions by independent routes. But I am not 

 aware that any one has presented the same analysis of cohesion, or of 

 the molecular constitution of material surfaces. Especially does the 

 derivation of evaporation from molecular mechanics seem to me novel 

 and worthy of careful consideration. Denny indicates deaeration as 

 a cause of steam-boiler explosions ; but it is essentially as an experi- 

 mental deduction, and not connected with its mechanical derivation. 



In conclusion, I will present an outline of a most interesting illus- 

 tration of creative design in the earth's co-ordination. The explana- 

 tion of evaporation which has been given, shows that for each fluid, 

 the formation of vapor lies within certain definite limits of temperature 

 as a result of its primary structure. These limits differ greatly in 

 different fluids. Now, in framing the earth for habitation, or for the 

 proper life of animal and vegetable forms, something equivalent to 

 rain was necessary from the constant descent of fluids to the lowest 

 level. Without some agency to lift the great organic fluids above the 

 ocean bed, sterility would have been the lot of all which rose above 

 its surface, and terrestrial organisms would have been quite impossible. 

 But fluidity does not involve evaporation except within certain definite 

 limits, special for each liquid. Again, evaporation might go on and 

 yet no capacity for condensation exist, except within other limits of tem- 

 perature quite unattainable save through special arrangement. Rain, 

 then, with our earth and atmosphere, involved a special constitution 



