Page:Popular Science Monthly Volume 16.djvu/648

 this "tendency"? We might see any number of bodies falling to the ground, and might frame a correct law of their motion, without having the remotest conception of their possessing that downward pressure which we at once recognize when we take a lump of lead or iron into our hands; and it is obviously on our cognition of this pressure, that our idea of weight or ponderosity is based. Now, the instrumentality through which we take cognizance of it seems to me to be threefold: In the first place, we have the sense of simple pressure on the tactile surface—as when, the hand passively resting on a table, a weight is laid upon it. Secondly, we recognize it by the sense of tension which we experience when a weight is attached to a pendent limb, and which we refer to the muscles and ligaments which are thus put on the stretch; or when, the hand resting on the top of a cylinder of glass placed over an air-pump, the air is exhausted from beneath, so as to make us feel the downward "pressure of the atmosphere." In these two cases, the mind is the passive recipient of the sensory impressions. But, thirdly, when we determinately lift a weight or hold it suspended by our hands, we experience, in addition to the sense of pressure and the sense of tension, a sense of effort, which we recognize as an immediate revelation of consciousness, not referable to any physical impression, but of the same kind as that which we experience in a purely mental act, such as the fixation of the attention. And a little consideration will, I think, make it clear that it is on this "sense of effort" in resisting downward pressure that our cognition of weight is essentially based.

For, in the first place, the continuance of a moderate pressure on the cutaneous surface, like other sensory impressions that become habitual, soon ceases to affect us sensorially; for we cognosce rather the changes in the states of our sense-organs than the states themselves. Or, again, we may suffer under a temporary or permanent paralysis of the cutaneous sense, that may prevent our feeling the contact of the body we are lifting or supporting; and yet, recognizing its downward pressure in other ways, we can put our muscles into action to antagonize it. But, secondly, this paralysis may extend to the muscular sense, so that the feeling of muscular tension is wanting as well as that of contact-pressure; and yet none the less can a weight be lifted or sustained by a conscious effort, provided that the deficiency of the guiding sensations ordinarily derived from the muscle itself is supplied by the sight. A woman whose arm is sensorially but not motorially paralyzed can hold up her child as long as she looks at it, and a man affected with the like paralysis of his legs can stand and walk while looking at his feet. But, thirdly, since the mental sense of effort is experienced in every determinate exercise of our muscular power, and is, as all experience teaches, a necessary condition of that exercise; since, again, it is proportioned to the exertion we put forth, and continues as long as that exertion is sustained—it is in this, and not in the cutaneous or