266 ON THE SENSES. 



would be impossible, or at least in the highest degree uncertain. The apprecia- 

 tion of pressures according to the intensity of the sensation is tolerably exact 

 and certain, as we shall presently show ; but the scale by which v/e measure is 

 in nowise to be compared with the weights from which, with the help of the 

 balance, we determine the precise numerical value of these same pressures. 

 We have no scale on which the mind can read the absolute value of a sensation 

 of pressure of a determinate intensity; we are able, indeed, of two successive 

 sensations to say which is the stronger, which the weaker ; we can decide 

 whether the intensity differs little or much, but we cannot express this difference 

 in numerals and prmiounce one sensation to be twice, thrice, or half as strong as 

 another with which it is compared. While undergoing its training, the mind 

 gradually learns to connect with different sensations the idea of corresponding 

 differences of intensity in the forces of pressure, and the different degrees of 

 sensation with the ideas of the corresponding forces become so stamped upon 

 memory that at a moment's warning we are in a condition to interpret every such 

 sensation and to refer it to a certain amount of outward pressure. We form for 

 ourselves, for instance, an accurate recollection of the sensation which arises 

 when the weight of pressure stands, as the balance announces, at the ratio of 

 1 : 2, and in other absolute proportions, and in this way attain a faculty of dis- 

 crimination, gradually becoming through practice more sensitive and certain, 

 for amounts and differences of pressure. If we inquire as to the manner in 

 which we essay the trial of weight by help of the sense of touch, an interest- 

 ing circumstance must be noticed. It has already been seen that, for the esti- 

 mate of weights we have in our muscles a second means to which we principally 

 recur for the trial of greater weights. We lift the object and estimate its grav- 

 ity by the intensity of the feeling of effort in the exerted muscles, after we 

 have gradually learned to interpret this feeling in reference to its intensity, as 

 in the case of sensations of pressure. If, therefore, we would employ the latter 

 only in the trial, we must exclude all aid of the muscular feeling ; for which 

 purpose the organ of proof, the hand, must be fully supported and left at rest 

 with its back on the support, while the weights to be essayed are laid upon the 

 palm. For comparison of two different weights the determination is simple 

 and easy, if both are laid at the same time on two different places of the hand 

 and the relative weight be estimated by the intensity of the separate sensations 

 simultaneously excited. But our judgment is more accurate and our power of 

 discrimination nicer if we deposit the weights, one after the other, on the same 

 place of the hand, in such manner that the second and actual sensation is 

 compared- with the first and simply remembered one, instead of two actual 

 and simultaneous sensations being compared as in the former instance; 

 indeed, the recollection of the previous sensation is so tenacious that no incon- 

 siderable interval may elapse between the two trials without rendering our 

 judgment as regards the first sensation uncertain, and the greater the difference of 

 the two compared weights, the longer may this interval be without prejudice to 

 the accuracy of our estimate. If we lay on the hand of an individual whose 

 eyes are bandaged or averted, so that the sight shall lend no assistance to the 

 judgment, first a weight of two ounces, and afterward on the same place 

 a weight of two and a half ounces, the lapse of two minutes will occasion 

 no error as to the greater heaviness of the second. If the weights be depos- 

 ited on the hand in immediate sequence we distinguish with certainty 

 . fourteen and fifteen ounces as being of different weight, and recognize 

 which is the heavier. We should observe, however, that the weights to 

 be compared must be deposited on the same place of the hand ; " were 

 we to place one of two equal weights on the finger-end, so rich in nerves, 

 the other on the palm, in which the nerves are so much rarer, we should cer- 

 tainly decide in favor of the preponderance of the former. Again : it id 



