448 THOUGHTS ON THE 



bodies are, they are the more impatient of pressure, the 

 more acutely sensitive to it, as it were ; so much so, that if 

 disturbed ever so little from their natural position, they 

 endeavour with great velocity of movement to free them 

 selves from its effect, and resume their original state. To 

 effect which, the several parts, beginning with the part 

 struck, successively propel one another, in the same way as 

 an external force, and keep up that motion vigorously ; hence 

 results a continuous and, though invisible, intense vibra 

 tion of the parts. And this we see exemplified in glass, 

 sugar, and similar brittle substances, which, if they be 

 divided by a blade or edged instrument, are, as it were, in 

 a moment broken down in other parts distant from the 

 line described by the blade. Which evidently proves that 

 the motion of pressure travels to the parts of these sub 

 stances successively. This motion pervading all the parts 

 of the body, and trying, as it were, their compactness, 

 causes the breaking down of that part, where, from the 

 structure, the cohesion is weak. Yet does not this motion, 

 though it agitates and permeates the whole, come into 

 view, till a visible break or divulsion of continuity takes 

 place. Again, we observe, if we happen to bend and com 

 press between the thumb and forefinger the two ends of a 

 wire, or bit of cane, or the harder part of a pen, (or similar 

 bodies which unite flexibility with a certain degree of elas 

 ticity), they anon spring from the hand. The cause of 

 which motion is evidently discernible not to be in the ex 

 tremities compressed by the fingers, but in the middle part, 

 which is the seat of forcible pressure, to relieve itself from 

 which, the motion comes into play. And in this instance 

 it is clearly shown, that the alleged cause of motion, the 

 impulsion of the air, is inadmissible. For here there is no 

 concussion to let in a rush of air. This is also proved by a 

 slight experiment, when we press the fresh and smooth ball 

 of a plum, drawing the fingers gradually together, and in 

 this manner let it go. For in that instance also compres 

 sion is substituted for percussion. But the most conspi 

 cuous effect of this interior motion is in the revolutions and 

 gyrations of missiles while flying. The missiles, indeed, 

 proceed onwards, but they make their progression in spiral 

 lines, that is, by straight-lined and rotatory motion toge 

 ther, and indeed this curvilinear motion is so fleet, and at 

 the same time so easy, and somehow so familiar to things, 

 as to excite a doubt in my mind whether it does not depend 

 on some higher principle. Yet I think that the cause of 



