AIMS, ETC., OF SCIENTIFIC THOUGHT. 95 



Now, let us take an example from the building of bridges and roofs. 

 When an opening is to be spanned over by a material construction, 

 which must bear a certain weight without bending enough to injure 

 itself, there are two forms in which this construction can be made, the 

 arch and the chain. Every part of an arch is compressed or pushed by 

 the other parts ; every part of a chain is in a state of tension, or is 

 pulled by the other parts. In many cases these forms are united. A 

 girder consists of two main pieces or booms, of which the upper one 

 acts as an arch and is compressed, while the lower one acts as a chain 

 and is pulled ; and this is true even when both the pieces are quite 

 straight. They are enabled to act in this way by being tied together, 

 or braced, as it is called, by cross-pieces, which you must often have 

 seen. Now, suppose that any good, practical engineer makes a bridge 

 or roof upon some approved pattern which has been made before. He 

 designs the size and shape of it to suit the opening which has to be 

 spanned ; selects his material according to the locality ; assigns the 

 strength which must be given to the several parts of the structure ac- 

 cording to the load which it will have to bear. There is a great deal 

 of thought in the making of this design, whose success is predicted by 

 the application of previous experience ; it requires technical skill of a 

 very high order; but it is not scientific thought. On the other' 

 hand, Mr. Fleeming Jenkin ' designs a roof consisting of two arches 

 braced together, instead of an arch and a chain braced together ; and, 

 although this form is quite different from any known structure, yet be- 

 fore it is built he assigns with accuracy the amount of material that 

 must be put into every part of the structure in order to make it bear 

 the required load, and this prediction may be trusted with perfect 

 security. What is the natural comment on this ? Why, that Mr. 

 Fleeming Jenkin is a scientific engineer. 



Now, it seems to me that the difference between scientific and 

 merely technical thought, not only in these, but in all other instances 

 which I have considered, is just this : Both of them make use of expe- 

 rience to direct human action ; but while technical thought or skill en- 

 ables a man to deal with the same circumstances that he has met with 

 before, scientific thought enables him to deal with different circum- 

 stances that he has never met with before. But how can experience 

 of one thing enable us to deal with another quite different thing ? To 

 answer this question we shall have to consider more closely the nature 

 of scientific thought. 



Let us take another example. You know that if you make a dot 

 on a piece of paper, and then hold a piece of Iceland spar over it, you 

 will see not one dot but two. A mineralogist, by measuring the an- 

 gles of a crystal, can tell you whether or no it possesses this property 

 without looking through it. He requires no scientific thought to do 

 that. But Sir William Rowan Hamilton, the late Astronomer-Royal 



1 " On Braced Arches and Suspension Bridges." Edinburgh, Neill, 1870. 



