268 NATURE'S TEACHINGS. 



that the mistake was only discovered by the workmen, who 

 heard the strokes of their companions' picks on their sides, and 

 not in front. Consequently, a great waste of time took place, 

 and the centre of the tunnel had to be made with a double 

 curve, like the letter S, and trains are obliged to slacken speed 

 until they have passed it. 



Those who have lived long enough to remember the current 

 literature of the past generation will call to mind the 

 ridicule that was cast upon the idea of a tunnel that should 

 pass under the Thames. That it would be useful if it could be 

 completed, no one ventured to doubt, but that such an idea 

 could be conceived by any one out of a lunatic asylum was 

 rather too much for the journalists of the day. However, the 

 tunnel was made, and so proved the theorists wrong on the 

 one side. And, when made, it was of very little use, which 

 proved them wrong on the other side. Now the proposal to 

 carry a submarine tunnel from England to France excites not 

 half the opposition that was elicited by the comparative child's- 

 play of a tunnel under the Thames. 



The only mode of laying down the lines on which the men 

 worked is by suspending very heavy balls to very fine wires, 

 and then, by means of delicate optical instruments, ascertaining 

 whether the wires are in line with each other. 



Familiar instances of the use of this principle may be seen 

 in the plumb-rule and level of the builder or carpenter. The 

 latter, with a base of ten feet in length, is often used by the 

 gardener when he wishes to lay the absolutely level lawns 

 that are required for our modern game of croquet, where the 

 hoops are scarcely wider than the balls, and the lawn has in 

 consequence to be nearly as level as a billiard table. 



I may here remark that the name plumb-rule is derived from 

 the Latin word plumbum, or lead, in allusion to the leaden 

 weight at the end of the string. The word " plumber " is due 

 to the same source, and signifies a worker in lead. 



THESE invaluable aids to the development of civilisation are 

 due to one principle, namely, that which we call Gravitation, 

 but which ought more properly to be termed Attraction, and 

 which attracts all parts of the earth towards its centre. We 

 are all familiar with the anecdote of !Newton and the falling 



