NATURAL PHILOSOPHY. 191 



radius which revolved about the earth nearly in one plane, would during 

 that revolution turn always the same plane to the earth. Now, would such 

 a body be described as revolving upon its axis during such a revolution ? 

 By many persons it would not be so described. But the moon is described 

 by astronomers as revolving about her axis in the course of every month. 

 What are the reasons for such a description ? The reasons are briefly these : 

 1. The moon, is not fastened to the earth rigidly, nor fastened at all. 2. 

 The moon being thus detached, the reference of the moon to the earth as a 

 centre of revolution is arbitrary. 3. The other celestial bodies which revolve 

 about centres also revolve about their axes, and the rule regarding them 

 as not revolving about then 1 axes when they turn always the same face to the 

 centre, would produce confusion : it would, for instance, compel me to say 

 that the earth revolves upon her axis 365^ times in a year, whereas with 

 regard to the fixed stars she revolves 366 times. Also, when a body 

 revolves about a centre turning always the same face to the centre, then is 

 mechanical force required to make it so turn ; but no mechanical force is 

 required to make it remain parallel to itself while it revolves round a centre. 

 1. The moon is not fastened to the earth rigidly, as the ancients supposed 

 when they invented the crystalline spheres as the mechanism by which the 

 heavenly bodies revolve, and by which they are connected with one another ; 

 and as the body representing the moon is fastened to the body representing 

 the earth in machines made by man. The moon in nature is entirely detached 

 from the sun, and the fact of her turning the same face to the earth does not 

 at all form the machinery of her monthly revolution. Hence it is ascribed to 

 a separate motion, her monthly revolution on her axis. 2. The reference of 

 the moon to the earth is arbitrary. The moon revolves about the earth, but 

 she revolves about the sun also. She revolves about the sun more than about 

 the earth ; for w^hen she is between the sun and the earth, her face is concave 

 to the sun and convex to the earth's orbit. There are, in some respects, 

 stronger reasons for regarding her as fastened to the sun than as fastened to 

 the earth. But in truth she is not fastened at all ; and the simplest way is to 

 regard her as quite detached, and to consider her motion by which she turns 

 her face different ways as quite separate from the motion by which she 

 revolves about any centre. 3. The other celestial bodies also revolve about 

 their axes, and especially the earth. All persons agree in thus expressing the 

 fact in the case of the earth ; and as there are 365 days in the year, the 

 earth revolves 365 times on her axis with reference to the sun. By doing 

 this she revolves 366 tunes on her axis with reference to the fixed stars. 



ON THE DISCOVEPvY, CONSTRUCTION, AND EXPLANATION OF THE 



GYROSCOPE. 



During the past year no little discussion has been occasioned by the re-ap- 

 pearance before the public of an old but beautiful mechanical toy, to which 

 the name Gyroscope has been generally applied. As the principles involved in 

 the motions of this arrangement have been the theme of much discussion, we 

 present the following articles as containing the essence of all that has been 



