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or appearances. The word is, however, hke many of our other 

 words, sometimes used in a very loose and indefinite sense, as, for 

 instance, a man says, I have a theory that the heat of the sun is 

 produced in such and such a way. Now, to me at least, it is quite 

 clear that in so speaking he really means nothing more than that 

 he has an opinion to that effect. Why he elects to dignify it with 

 the name of a theory I cannot say, but so it is. Yet if the meaning 

 of the word opinion be what I take it to be, that is, the uncertain 

 judgment of the mind on any matter submitted to it, I think it 

 must be clear to you all that, in the present state of science, the 

 so-called theories of the sun's heat, as well as many other such 

 theories, are really nothing more than opinions, which may be 

 either right or wrong, as is the case with all opinions. If they are 

 proved to be right, why they will be opinions no longer but facts ; 

 if they are proved to be wrong they will be at once rejected. Now, 

 what do we know about the sun, with all our knowledge of it, to 

 enable us to say with certainty, that its heat is produced either this 

 way or that way ? if but little, then our theories are only opinions 

 with another name. The only definite sense in which the word 

 theory is used, is where we say that the theory of a certain opera- 

 tion or result is so-and-so. For instance, we say that the theory of 

 the ascension of a column of water in a tube that has been 

 exhausted of air, is — first, that water presses equally in all 

 directions ; secondly, that action and reaction are equal ; and, 

 thirdly, that at the sea level water is everywhere overlaid by a 

 stratum of air which exerts on it a pressure of nearly fifteen pounds 

 per square inch. These three stages in the explanation are facts 

 which have been established independently, and from them it 

 follows that if the atmospheric pressure on the water be anywhere 

 removed, as it would be under an exhausted tube, since action and 

 reaction are equal, and water presses equally in all directions, then 

 the surface of the water under the tube being reheved of downward 

 pressure must rise in that tube until the weight of the ascending 

 cohimn is equal to the pressure of the air that has been removed. 

 Now, every part of this explanation or theory is capable of 

 demonstration, in other words can be shown to be a fact, and 



