450 



NATURE 



[Sept. 5, 1889 



unchanged in speed or in direction, we feel nothing, as has been 

 pointed out already. Let us look at the case when the speed 

 changes, the direction remaining the same. We have to consider 

 separately three different directions : (i) horizontal, (2) up, and 

 (3) down, because we shall see that our sensations are different 

 in these three cases. With change of speed in horizontal motion 

 we are all familiar. The starting and the stopping of a train or 

 of a steamer give us ample means of studying it. We all know 

 the jolt of a badly started train. What we feel in such a case is, 

 mainly at all events, due to our body being jerked forward or 

 backward, according as we are sitting with our back or our face 

 to the engine. But if the train is carefully started we find that 

 our rate of motion may in a very short time be changed from 

 nothing (relatively to the earth) to, say, thirty or forty miles an 

 hour, without our feeling anything but the up and down rattle due 

 to the slight unevenness of the rails. And the same was the case 

 till comparatively lately with stopping. Now, however, since the 

 introduction of the continuous brake, a train can be so rapidly 

 stopped — its rate of motion, that is our late of motion when we 

 are in it, can be so very quickly changed from, say, sixty miles an 

 hour, to nothing — that we do feel a strange, not altogether pleasant 

 sensation. Experience has taught us what that sensation means, 

 but at first it was so novel that experience was necessary to in- 

 terpret it. It is not a sensation of jolt, the change though rapid 

 is not abrupt. What we really feel, although it takes some 

 amount of careful observation and some thought to see this 

 clearly, is that the direction of the vertical, the direction in which 

 a body falls, the direction in which our body presses has been 

 changed. We feel this most distinctly if we are standing when 

 the brake is applied ; we feel that if we do not take means to 

 prevent it, we shall fall over, and we prevent this by bringing 

 our body into the line of the new vertical. Our feeling of un- 

 steadiness depends on our uncertainty how long the new state of 

 matters is to last. It lasts as long as the speed is being changed 

 at the same rate, and the deviation of the new from the real 

 vertical depends on the rate at which the speed is being changed. 

 Our perception of deviation from the vertical is pretty acute. 

 Most of us can tell a line to be off the vertical when it is inclined 

 only a few degrees to it. In ordinary cases we have extraneous 

 help in judging. We have walls or chimneys, known to be 

 vertical, or surfaces known to be level, with which to compare, 

 but even when we have no assistance of this kind we are not 

 often far wrong. It might be supposed that it is the pressure of 

 our body on the floor or ground that gives us the idea of the 

 vertical, but that idea still exists in cases where we can feel no 

 such pressure. If our body is supported in water, or entirely 

 submerged, as in diving, we still have a very distinct, and fairly ac- 

 curate notion of up and down, although in such cases, as our body 

 is very nearly of the same density as water, the resultant pressure 

 on it is small. We shall see in a little a possible explanation of 

 our sense of the vertical. When the train in which we are 

 travelling runs quickly round a sharp curve, we feel something 

 very like the sensation just described. And indeed it is due to a 

 perfectly similar cause ; the apparent vertical is the direction of 

 the resultant of the force of gravity and the centrifugal force, and 

 is, as every engineer knows, more inclined to the real vertical 

 as the curve is sharper and the speed of the train greater. 

 In this case the sensation is complicated, because the motion is 

 not one purely of translation, but, as already pointed out, is 

 compounded of translatory and rotational motion. 



Let us now look at cases of up and down motion. As we can 

 study horizontal motion in the railway train, so we can up and 

 down motion in a lift. Here also we see that it is change of 

 motion which we really perceive. For, once the lift is started, 

 and is moving smoothly and uniformly, either up or down, we 

 are quite unconscious of the mction. It is the start and the stop, 

 or the quickening or slowing of the motion only that we feel. 

 And the stopping of the upward motion produces exactly the same 

 feeling as the starting of the downward motion, provided they are 

 equally smooth and free from jerk. It is easy to see what are 

 the physical conditions here. Just as the acceleration in a 

 horizontal direction inclines the apparent direction of gravity, fo 

 acceleration up increases, and acceleration downwards diminishes, 

 the apparent intensity of gravity. If the lift fell down, un- 

 restrained, its inmates, during the short time the experiment would 

 last, would have no sense of the force of gravity at all. An object 

 dropped from the hand would not fall down to the floor, because 

 the floor itself would be falling at the very same rate as the object. 

 And what is true in this extreme case is true also in a measure in 

 all cases of downward acceleration]. But only in cases of accelera- 



tion, for, however fast the lift goes dow n, if it moves uniformly, 

 without change of speed, the bodies of those in it press on its 

 floor exactly as if it were at rest. Similarly, upward acceleration 

 increases the apparent force of gravity. The physical conditions, 

 then, of our perception of acceleration of translatory motion in 

 any direction are change in the apparent direction, intensity, or 

 both, of the force of gravity. It is a ^trange and interesting fact 

 that our perception of downward acceleration— that is, of 

 diminished force of gravity — is more acute than that of accelera- 

 tion upward or in a horizontal direction. We feel the starting 

 of the lift as it goes down, and its stoppage when it has come up, 

 much more distinctly than the start on the way up or the stop at 

 the end of the journey down. And when we are rocked in a 

 rolling steamer, it is the beginning of the downward move that 

 is most perceived. 



Having now discussed the phenomena of our sensations con- 

 nected with translatory motion, let us examine what our experience 

 is when we are turned round, or subjected to rotational motion. 

 We execute such movements every minute of our waking life. 

 But as with tranylational motion, so, and even more, with rota- 

 tional motion, it is impossible to analyze our sensations \\hen 

 they are complicated with what we feel we do. And in the case 

 of rotation a very serious complication is introduced by our seeing 

 how we are being moved. So that, to make a strict examination 

 of our sensations in this matter, the observer must place himself 

 blind-folded on the rotating apparatus, and be passively turned 

 round. Or, as in Prof. JMach's very ingenious experiments, a small 

 hut with translucent paper windows may be placed on the turn- 

 table for the reception of the observer. Just as we can move, or be 

 moved, right or left, backwards or forwards, up or down, so we 

 can be spun round about a fore-and-aft, a right-and-left, or an 

 up-and-down axis, and about each of these axes either the one or 

 the other way round. It is plain, however, that we can get simple 

 results only in the case of rotation about a vertical axis, because 

 otherwise a great complication would be introduced by the vary- 

 ing position of our body relatively to the diiection of gravity. 

 VVe shall see that we can get everything we require with rotation 

 about a vertical axis. Here we find, as in the former case, that 

 it is only change of motion that is perceived. The observer sits 

 on a chair on the turn-table, his eyes are bandaged, and an 

 assistant gives the table a steady, uniform rotation. At first the 

 observer feels the turning quite distinctly, but after less than a 

 whole revolution the sensation becomes very indistinc% and, 

 while the turning still continues at the same rate, soon disappears 

 altogether. If the rate of turning is now increased, he feels it 

 begin again, all that he perceives being the increase ; his per- 

 ception of that also soon dies away, so that in a short time he 

 may be spinning rapidly round, while he feels completely at rest, 

 and is only aware that he has been gently turned round a little, 

 two or three times. But if you now stop him he feels a turning 

 round in the opposite way to that in which he really was turning, 

 the fancied rate of turning being, at the moment of stopping, that 

 of the real turning which has just been stopped. This imaginary 

 rotation dies away, exactly as the sensation of the real rotation 

 did. Now, v.ry nearly the same thing takes place, whatevtr is 

 the position in which the head is placed during the experiment, 

 if the head is kept rigidly in the same position during the whole 

 of the experiment. Let us look at a case in which the position 

 of the head is not kept the same during the experiment. The 

 observer sits on the table, with his head irclined to one side, so 

 that the line from ear to ear is vertical. He is now turned 

 uniformly round ; as before, he feels the turning at fiist, but as 

 the uniform turning goes on the sensation dies away. When he 

 feels perfectly at rest, let him give the word to stop, and at the same 

 instant raise his head into the ordinary position. He will now 

 feel as if he were being turned about the line from ear to ear — 

 that is, now, about a horizontal axis. If his right ear was down 

 when he was actually being rotated, and if the turning was with 

 the hands of a watch lying with its face up, then the imaginary 

 rotation will be the opposite way round — he will feel as if his 

 head were going forward and his feet back. This sensation will 

 last only a .> hort time, but there is a risk in trying the experiment, 

 that the observer may try to correct this alarming overturn by 

 throwing himself backwards ; if he is nervous it may therefore 

 be as wtll to have him strapped to the chair. Whatever line in 

 the head we make vertical while the leal rotation is going on 

 — that is, whatever line in the head we make the axis of the real 

 rotation — that line is the axis of the apparent rotation which we 

 feel when the real rotation stops, however we may move our 

 head at the time of the stoppage. Theie is a practical joke 



