SCIENCE, INVENTION, DISCOVERY. 



359 



of the receiving disc through the medium of 

 varying impulses of electricity sent to the mag- 

 net of the latter by the magnet of the former. 

 The movements of the former are controlled 

 by the human voice. It follows that the move- 

 ments of the latter are also controlled by the 

 same voice. It is the reversal of a process. If 

 a disc is moved by the voice in a certain way, 

 the moving of a disc in the same way by some 

 other means will imitate the voice. So far as 

 Volume is concerned the imitation is micro- 

 scopic. But it can be heard, and answers all 

 purposes. It is doubtful if, among all the 

 productions of human genius, there will ever 

 be anything nearer the miraculous than the 

 almost universally used telephone. 



Telescopes. This invention is noticed 

 by Leonard Digges, about 1571. Roger Bacon, 

 A. D. 1250, described telescopes and micro- 

 scopes exactly, and yet neither was made till 

 one Metius, at Alkmaar, and Jansen, of Mid- 

 dleburg, made them about the same time ; the 

 latter from an accidental discovery made by his 

 children, 1590-1609. Galileo imitated their 

 invention by its description, and made three in 

 succession, one of which magnified a thousand 

 times. With these he discovered Jupiter's 

 moons, and the phases of Venus. Telescopes 

 became very popular, and were improved by 

 Zucchi, Huygens, Gregory, and Newton ; and 

 finally by Martin, Hall, Dolland, and Herschel. 

 Achromatic telescopes were made by Hall about 

 1723. Many excellent and powerful tele- 

 scopes have since been constructed. 



Terms in Electricity. The technical 

 terms used in regard to electricity refer to 

 units of various nature. Thus the unit of 

 capacity is one farad ; the unit of activity, one 

 watt ; the unit of work, one joule ; the unit of 

 quantity, one coulomb ; the unit of current, 

 one ampere ; the unit of resistance, one ohm ; 

 the unit of magnetic field, one gauss ; the unit 

 of pressure, one volt ; the unit of force, one 

 dyne. The names are mostly derived from the 

 names of men that have been famous in the 

 field of electrical research. Thus Michael Far- 

 aday, James Watt, and James P. Joule, famous 

 English discoverers, give their names to the 

 first three units mentioned; Charles A. Cou- 

 lomb and Andre M. Ampere, French invent* 

 ors, to the two units following ; G. S. Ohm 

 and Carl F. Gauss, Germans, name two more 

 units ; and the volt is named from the Italian 

 discoverer, Volta. The dyne is derived from 

 the root word of dynamo, itself meaning force. 



Thermometer, The. The thermometer 

 is an instrument for measuring the heat or tem- 

 perature of bodies by the regular expansion of 

 mercury or alcohol in a graduated glass tube. 

 Halley proposed the substitution of mercury 



for alcohol in 1697- The thermometers usually 

 employed are Fahrenheit's, the Centigrade, 

 and Reaumur's, the first invented in 1726, 

 and the two others soon afterwards. 



The following table is interesting as a com- 

 parison of the three thermometers : 



Ice mcUsat 32; temperature of globe, 50 ; 

 blood heat, 98 ; alcohol boils, 174 ; water 

 boils, 212 ; lead melts, 594 ; heat of common 

 fire, 1,140; brass melts, 2,233; iron melts, 

 3,479. 



Thunder is caused by the sudden re- 

 entrance of the air into a vacuum which is sup- 

 posed to be caused by the lightning in its pas- 

 sage through the atmosphere. The electricity 

 exerts a powerful repulsive force upon the par- 

 ticles of air along the path of its discharge, 

 thus making a momentary vacuum. Into this 

 void the surrounding air rushes with a violence 

 proportioned to the intensity of the electricity, 

 and is thus thrown into vibrations, which are 

 the source of the sound. 



Tides, The The ebb and flow of tidal 

 waters depend upon the moon to a great extent. 

 Twice every day we have the tides, twelve 

 hours apart, and the flow and ebb are merely 

 examples of the attraction of gravitation which 

 is exercised on all bodies, whether liquid or 

 solid. The tides may be compared to a great 

 wave, which, raised by the moon's attraction, 

 follows her in her course round the earth. The 

 sun also aids in this effect, but as the moon is 

 so much nearer the earth her influence is far 

 greater. The tides are highest at the equator 

 and lowest at the poles, because the tropics 

 are more exposed to the lunar attraction. 



Tobacco. The name tobacco is thought 

 by some to have been taken from Tobacco, a 

 province of Yucatan ; by others from Tobago, 

 an island in the Caribbean Sea ; and by still 

 others from Tobasco, in the Gulf of Florida. 

 The plant, although it is asserted that the 

 Chinese have used it from earliest times, 

 was not introduced into Europe until after the 

 discovery of America by Columbus. lie first 

 found it in use on the Island of San Domingo 

 in the West Indies. The Indian, among all 

 the tribes from T*eru to Upper Canada, smoked 

 it in pipes. Tht, seed of the plant was first 

 introduced in Europe by Gonzalo Hernandez 



