Telegraph

wires apparently rise and fall because the weight of the wire makes it dip between the posts, no wire being strong enough to bear the strain of being stretched quite level. Consequent!}', the course of the wires is a succe ion of curves, with the convex downwards. The optical illusion is, therefore, due to the fact that the eye pa es them so quickly that they appear to be moving when it is really the train that moves. A similar illusion may be found when a field in furrows or with a|crop in lines is at right angles to the railway, when the furrows or lines of crop will appear to be revolving like the spokes of a wheel. i860, but was made practical by Graham Bell in 1876. It consists of a wooden box, within which is an iron vibrating plate fixed in front of a steel magnet, round which there is a coil of insulated copper wire. The two ends of the wire are connected by two united wires to the corresponding instrument, and the vibrating plate is covered with a mouth-piece. The microphone and the phonograph are simply modifications of the same principle. Bacon hail had a very definite idea of the principle involved. The children of a spectacle-maker in Holland, called Jansen, were playing one day with two powerful magnifying gla es and happened to place one below the other in such a position that a church tower in view seemed suddenly to come much nearer to them. They told their father, and he made up several * telescopes ’ as toys. Galileo heard of this and had soon perfected it into the modern telescope, though his first instrument only magnified three times. He soon made another which magnified nine times, and with it he discovered Jupiter's moons. Nowadays, instead of rough instruments constructed and worked under great difficulties, like the telescopes of Galileo and Herschel, we have magnificent refracting cnes, with large achromatic lenses, and they are sen registering, recording in a permanent form phenomena which had formerly to be recorded at intervals by human observers who were liable to error. Now clockwork regulates the movements of the telescope itself, the microscope enables the observer to read off the most delicate measurements, and photography gives permanent records of pa ing phenomena.