TOOLS OF MEASUREMENT. 267 



as that of sand-paper. The reason is this. The flinty points 

 of the Equisetum are set upon parallel ridges something like 

 those of a file, while the scales of the Dog-fish are without any 

 apparent order, being crowded against each other like the cutting 

 particles upon the sand-paper. That there should not be an 

 order, and that a definite one, is out of the question. But it 

 has not yet been detected by human eyes, and therefore may be 

 practically treated as non-existent. 



Tools of Measurement. 



In many of the arts, more especially those which belong to 

 engineering and carpentering as a part of architecture, it is 

 absolutely necessary to make sure of a perpendicular line, 

 i.e. a line which, if continued, would reach from any point of 

 the earth's surface to its exact centre below and its zenith 

 above. Were it not for the power of producing this line, none 

 of the great engineering works of modern or ancient days 

 could have been undertaken. 



Take, for example, the wonderful tunnels which have been 

 driven through the earth, of which the Mont Cenis Tunnel is 

 one of the greatest triumphs of modern engineering. Begin- 

 ning, as the workmen did, at opposite ends of a tunnel many 

 miles in length, and labouring only by the lines laid down by 

 the engineers, the men worked steadily on until they met in 

 the centre. 



A few blows, and the then narrow dividing wall was shattered, 

 the men shook hands through the aperture, and then, after 

 enlarging it, leaped wildly from one side to the other, having 

 succassfully solved the great problem. With such marvellous 

 precision had the lines been laid, that only a few inches had to 

 be smoothed down on either side, and the sides or walls of the 

 tunnel showed no traces of the junction. 



So rapid has been the progress of engineering that a 

 tunnel of a mile in length would, within the memory of man, 

 have been thought as daring a project as was the Mont Cenis 

 Tunnel, which has just been given as an example. Indeed, I 

 know of a railway tunnel, not quite a mile in length, where 

 the engineers had committed some error, so that the two halves, 

 instead of meeting exactly, overlapped each other so much 



