150 Ma. E. Hunt on certain Phenomena connected with 



the principle of the parallelogram of forces. If we apply such an impulse 

 as would have made a similar body, in a state of rest, move in the desired 

 direction, it will not produce the desired result, for the body will move 

 in a direction lying between that of the original motion, and that corre- 

 sponding to the new impulse. Now, it is something very like this that 

 occurs with a rotating body. In trying to change the plane of motion 

 we endeavour to give new directions to the particles already in motion 

 in other directions, and they will tlierefore not move in the directions 

 in which the new impulse tends to move them, but in intermediate 

 directions. In other words, the new impulse will not make the body 

 rotate about the same axis that it would have done, had it been at rest. 

 It will, however, make it rotate about a new axis different from its 

 original one — and the plane of motion will really be changed to the full 

 extent of the new impulse imparted. 



Professor Piazzi Smyth read a paper before the Royal Scottish Society 

 of Arts, in 1856, on the angular disturbances of ships, in which he 

 described his admirable, ingenious, and correctly designed apparatus, to 

 be used on shipboard to give perfect steadiness to a telescope and 

 observer, notwithstanding the most violent movements of the ship. In 

 an appendix to this paper, and in treating of the parallelism of the 

 earth's axis, he appears to consider that the dim-nal rotation of the 

 earth is necessary to maintain the inclination of the axis — that it is the 

 resistance due to the diurnal rotation that causes the axis to maintain 

 a uniform inclination. Now, I think it is plain from what I have said, 

 that the rotation would not maintain the inclination of the earth's axis, 

 were any force applied to change that inclination — it would merely 

 cause the change of incUnation to take place in a different direction from 

 that of the disturbing force. But as regards the earth itself, it seems 

 to me that before we search for some power to maintain the inclination 

 of its axis against the action of disturbing forces, we must first show 

 the necessity of such a power by proving the existence of disturbing 

 forces equal to the task of altering the inclination.* 



Professor Smyth, referring in the same paper to an incorrect explana- 

 tion of the gyroscope, says, " That the true explanation of the phenomena 

 may be obtained by observing everything that we see, instead of confin- 

 ing our attention to one favourite point only out of many. Examining 

 the experiment again,' ' he continues, " in this spirit, we find that at the 

 same time that the spinning of the wheel serves to restore the apparent 

 balance, a horizontal motion of it about its central point of support 

 takes place ; and this, which escaped the attention of the paradox-finder, 



• In the particular observations of Prof. Smyth, to which these remarks refer, I do not 

 understand him to be considering anything connected with the precession of the equinoxes. 



