234 PLEASANT WA YS IN SCIENCE. 



equalled many feet* On the other hand, it is possible to 

 have a considerable practical experience of scientific instru- 

 ments without sound knowledge of the principles of their 

 construction ; insomuch that instances have been known in 

 which men who have effected important discoveries by the 

 use of some scientific instrument, have afterwards obtained 

 their first clear conception of the principles of its con- 

 struction from a popular description. 



It may be well to consider, though briefly, some of the 

 methods of communication which were employed before 

 the electric telegraph was invented. Some of the methods 

 of electric telegraphy have their antitypes, so to speak, in 

 methods of telegraphy used ages before the application of 

 electricity. The earliest employment of telegraphy was pro- 

 bably in signalling the approach of invading armies by 

 beacon fires. The use of this method must have been well 

 known m the time of Jeremiah, since he warns the Ben- 

 jamites " to set up a sign of fire in Beth-haccerem," because 

 " evil appeareth out of the north and great destruction." 

 Later, instead of the simple beacon fire, combinations were 

 used. Thus, by an Act of the Scottish Parliament in 1455, 

 the blazing of one bale indicated the probable approach 

 of the English, two bales that they were coming indeed, 

 and four bales blazing beside each other that they were in 

 great force. The smoke of beacon fires served as signals 

 by day, but not so effectively, except under very favourable 

 atmospheric conditions. 



Torches held in the hand, waved, depressed, and so 

 forth, were anciently used in military signalling at night ; 

 while in the day-time boards of various figures in different 

 positions indicated either different messages or different 

 letters, as might be pre-arranged. 



Hooke communicated to the Royal Society in 1684 a 

 paper describing a method of " communicating one's mind 



* The instrument was lent to Mr. Huggins by Mr. W. Spottis. 

 woode. It has been recently employed successfully at Greenwich. 



