162 THE POPULAR SCIENCE MONTHLY. 



to advantage. With the aid of such a cheap method of projection, a 

 grammar school master can give quite an extended course in physics 

 with simple apparatus. He can draw his own diagrams on smoked 

 glass, fixing the drawing by exposing it to the vapor of alcohol, which 

 is evaporated from a shallow dish ; and for the money which is ex- 

 pended for a lime-light apparatus enough apparatus can be bought 

 which, supplemented by a water or a kerosene lantern, would illustrate 

 a full course of elementary lectures on physics. In many school col- 

 lections of apparatus, a few expensive instruments will be found : an 

 air-pump ; a Holtz electrical machine ; a large induction coil. One 

 or two of such instruments form the rallying point of the department 

 of physics, and are accompanied by meager and disjointed apparatus. 

 The student collects, so to speak, his thoughts about the picture of a 

 complicated machine ; his ideas of the pressure of gases or rarefied air 

 are complicated by the imperfect remembrance of certain valves. Elec- 

 tricity of high tension means something evoked by an electrical ma- 

 chine. These pieces of apparatus which I have mentioned form a 

 salient point of attack upon the system of instruction in physics too 

 common in many schools. A good air-pump is difficult to keep in 

 order, and finds its true place only in the private laboratory of an in- 

 vestigator, or in a college collection of apparatus. In the secondary 

 grade of schools some form of Sprengel's pump, or, where there is an 

 available head of water, an aspirator, will illustrate varying pressures 

 sufficiently well. The new Holtz machine which schools are anxious 

 to possess can only serve as a toy, for the theory of its working is 

 very hard to comprehend even by those who have studied the subject 

 in mature years. 



The modern view of the physical universe is that there is no such 

 state as rest : the particles of a gas are in an incessant state of motion, 

 and it can be maintained that when a stone rests upon a table it is not 

 at rest ; for it is forced downward by the action of gravitation through 

 a very small distance, and the elasticity of its support tends to move 

 it upward through the same distance. The term statics is apt to be 

 misleading, and the best- writers on science of to-day begin treatises 

 on natural philosophy with the subject of dynamics or forces in mo- 

 tion. In no subject, however, is the division into statics and dynamics 

 so illogical as in the subject of electricity. In most schools a stu- 

 dent begins the study of this subject with frictional electricity and 

 the electrical machine. An advanced student in a university pursues 

 the opposite plan, and approaches the subject, even if it be for the 

 first time, from the standpoint of the voltaic cell, and traces the devel- 

 opment of the force up to the point of the generation of electricity 

 similar to that produced by an electrical machine. Very little knowl- 

 edge can be obtained from the exhibition of toys like dancing pith- 

 balls, insulated stools, miser's plates, and apparatus for obtaining 

 shocks. 



