CORRESP ONDENCE. 



617 



cular motion, is impressed upon it. " Cen- 

 trifugal force" is a misnomer a convenient 

 fiction to represent resistance. Resistance 

 or inertia only opposes motion ; it never 

 produces it ; and is, therefore, not force. 

 Hence any explanation of phenomena that 

 assigns " centrifugal force " as the real 

 cause in producing motion or change of 

 motion is wholly erroneous, and subversive 

 of Newton's first law. No such resort to 

 " centrifugal force " is necessary in the ex- 

 planation of the tides. 



I must confess entire ignorance of the 

 experimental demonstration that bodies 

 weigh more or are heavier at midnight than 

 at any other hour of the twenty-four. If 

 that be true, and the cause assigned by Prof. 

 Schneider a sufficient explanation, then the 

 lunar midnight should produce the same ef- 

 fect as the solar. On this point allow me 

 to quote from Sir William Thomson and 

 Prof. P. G. Tait's " Treatise on Natural Phi- 

 losophy," vol. i., page 662. The authority 

 of these physicists must be acknowledged : 

 " Hence as the moon or anti-moon (an im 

 aginary moon 180 from the real one) rises 

 from the horizon to the zenith of any place 

 on the earth's surface, the intensity of appar- 

 ent gravity is diminished by about bijootnni 

 part ; and the plummet is deflected toward 

 the point of the horizon under either moon 

 or anti-moon by an amount which reaches 

 its maximum value when the altitude is 45. 

 The corresponding effects of solar influence 

 are of nearly half these amounts." 



Does Prof Schneider mean to subvert 

 Newton's third law that action and reaction 

 are always equal ? If he can prove that the 

 centripetal force for any point of a revolv- 

 ing body is greater or less than its reaction, 

 the " centrifugal force," he will certainly 

 disprove Newton's law, and compel a recon- 

 struction of most, if not all, mechanical 

 propositions. 



His statements respecting the value of 

 " centrifugal force " (properly centripetal 

 force or acceleration toward the centre) as 

 depending on the radius of curvature are in- 

 correct. It is not unconditionally true that 

 " in a short curve the centrifugal force is very 

 great." On the contrary, if the time of revo- 

 lution is constant, the " centrifugal force " 

 varies directly as the radius, increasing as 

 the radius increases. If the velocity of ro- 

 tation is constant, " centrifugal force " va- 

 ries inversely as the radius, increasing as 

 the radius decreases. Neither of these con- 

 ditions is met in the comparison of the revo- 

 lutions of the earth and the moon in their 

 orbits, since neither times nor velocities are 

 the same in the two orbits. In fact, the 

 acceleration of the earth (and, therefore, of 

 the moon) toward the sun is about -,'0^0 of 

 an inch ; while that of the moon toward the 

 earth is a little less than -y^^ of an inch a 

 second. The acceleration of the earth tow- 



ard the common centre of gravity of earth 

 and moon is only a small fraction of the 

 moon's acceleration toward the same point. 

 These accelerations are the measures of " cen- 

 trifugal force." Hence, according to Prof. 

 Schneider's theory, the solar tide should be 

 many times greater than the lunar. 



My amazement reaches a climax when I 

 read, near the close of the article in ques- 

 tion, that " centrifugal force acts in a line 

 tangent to the earth's orbit; " or, " in a di- 

 rection at right angles with the radius-vec- 

 tor." Really, Mr. Editor, your compassion 

 should have saved Prof Schneider from mak- 

 ing such an egregious blunder. 



In reference to the true explanation of 

 the tides, the length of this communication 

 will allow me to add only that, if Prof. 

 Loomis, in his admirable " Treatise on As- 

 tronomy," had applied to the tides the same 

 explanation and figure, mutatis mutandis, 

 that he uses in estimating the amount of 

 the sun's disturbing effect on the moon's 

 motion, no uncertainty would remain in the 

 mind of teacher or student respecting the 

 cause of the tides. 



H. S. Carhart. 



Northwestern TJiavERSiTT, 

 EvANSTON, Illinois, July 2, 1877. 



.\ 



"THE ZODIACAL LIGHT." 



To th e Editor of the Popular Science Monthly. 



Prof. Brame's article on " The Zodiacal 

 Light," in The Popular Science Monthly 

 for July, may make a recent observation 

 of that phenomenon of interest. About 

 eight o'clock in the evening of July 3d my 

 attention was called to a peculiar appear- 

 ance of the sky. The sun had been below 

 the horizon about an hour. From the point 

 in the horizon where it was last seen a 

 broad band of pink or rose-colored light 

 followed the ecliptic across the sky to the 

 opposite horizon. Its south limit was sharp- 

 ly defined its intensity nearly the same 

 from horizon to horizon. Its north limit 

 was not determinable, the pinkish light ex- 

 tending nearly or quite to the horizon, fill- 

 ing the entire northern sky. The southern 

 sky from the ecliptic was of the normal 

 blue color, with the exception of a single 

 streak of a darker blue, extending from the 

 point where the sun sank below the horizon 

 about 90 into the southern sky, making an 

 angle of say 30 with the southern limit of 

 the rose-colored light, or the ecliptic. 



There were none of the auroral charac- 

 teristics. The light was steady, and the 

 entire exhibition as described lasted for 

 twenty minutes, when it all faded away 

 gradually, leaving a perfectly clear sky, with 

 only a trace of the pink in the west. 



Charles A. Morey. 



"Winona, Minnesota, July 5, 1877. 



