6i8 THE POPULAR SCIENCE MONTHLY. 



this " tendency " ? We might see any number of bodies falling to the 

 ground, and might frame a correct law of their motion, without hav- 

 ing the remotest conception of their possessing that downward pres- 

 sure 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 instrumen- 

 tality through which we take cognizance of it seems to me to be three- 

 fold : 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/ee^the downward "pressure of the atmosphere." In these 

 two cases, the mind is the passive recipient of the sensory impressions. 

 But, thu'dly, 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 se7ise of effort, which we recognize as an itnme- 

 cUate revelation of consciousness, not referable to any physical impres- 

 sion, 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 consid- 

 eration will, I think, make it clear that it is on this " sense of effort " 

 in resisting downward pressure that our cognition of weight is essen- 

 tially based. 



For, in the first place, the continuance of a moderate pressure on 

 the cutaneous surface, like other sensory -7mpressions that become ha- 

 bitual, 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 antago- 

 nize 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 sus- 

 tained 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 ex- 

 perience 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 



