Polytechnic Association. 877 



in seventeen, up which the engine took its way steadily at three 

 miles per hour. The road had been recently covered with loose soil, 

 but that did not cause the rubber tires to slip. On the levels the ave- 

 rage speed was eight miles per hour. The trial was, on the whole, 

 satisfactory ; and when it is considered that the engine and omnibus 

 do the work of twenty ordinary carriages, forty horses and twenty 

 men, at a fifth of the cost, the system may be expected to prove 

 advantageous." 



Stellee Motion Illttsteated. 

 Mr. Robert Weir read the following notice of Prof. Mayer's lecture, 

 at New Haven from the College Courant : 



Acoustic illustration of the method hy which stellar motions are 

 determined with the spectroscope. 



The fourth of the series of lectures known as the Mechanics' Course,, 

 was delivered on Thursday evening last, in the large hall of the 

 Sheffield Scientific School, upon the above subject, the lecturer being 

 Prof. A. M. Mayer, of the Stevens' Institute of Technology. 



Prof. Mayer began by calling the attention of his audience to the 

 character of vibrations, instancing the pendulum as one of the best 

 examples of visible mass-vibration, and saying that the curve repre- 

 senting its motion was that representing all other vibratory motions 

 of whatever kind. The curve he had obtained experimentally by 

 means of ah ingenious apparatus which he described. Besides these, 

 there are molecular vibrations due to elasticity, the action of which 

 was very clearly illustrated upon the blackboard. A water-wave is a 

 mass-vibration, as is shown admirably by Prof. Lyman's wave-apparatus. 

 The progressive character of a wave was then exhibited by means of 

 a long wire coil, along which an impulse was transmitted as a visible 

 undulation or wave. As an example of a molecular vibration due to 

 elasticity, the vibration of a Brown & Sharp's straightedge, fastened 

 firmly at one end, was given ; and a series of beautiful curves drawn 

 upon smoked glass by a wire attached to such a vibrating rod, were 

 thrown on the screen. 



The lecturer then passed to the theories of light, describing the 

 emission theory and the undulatory theory. Certain phenomena, 

 such as reflection, refraction and dispersion, could be equally well 

 accounted for by either; but certain others, such as those of inter- 

 ference, could be explained only by a wave or undulatory theory. 

 This latter theory supposes a trembling of the particles, either of air 



