CHAELES BABRAGE. 189 



should practice each adjustment separately, aud write down the results 

 wherever he cau measure its deviations. 



Having thus practiced the adjustments, the next step is to make an 

 observation. But in order to try both himself and the instrument, let 

 him take the altitude of some fixed object, a terrestrial one, and having 

 registered the result, let him derange the adjustment, and repeat the 

 process fifty or a hundred times. This will not merely afford him 

 excellent practice, but enable him to judge of his own skill. 



The first step in the use of every instrument is to find the limits in 

 which its employer can measure the same object under the same circum- 

 stances, and, after that, of different objects under different circumstances. 



The principles are applicable to almost all instruments. If a person 

 is desirous of ascertaining heights by a mountain-barometer, let him be- 

 gin by adjusting the instrument in his own study, and, having made the 

 upper contact, let him write down the reading of the vernier, and then let 

 him derange the upper adjustment only, re-adjust, and repeat the reading. 

 When he is satisfied about the limits within which he cau make that 

 adjustment, let him do the same repeatedly with the lower, but let him 

 not, until he knows his own errors in reading aud adjusting, pronounce 

 upon those -of the instrument. In the case of a barometer, he must also 

 be assured that the temperature of the mercury does not change during 

 the interval. 



A friend once brought me a beautifully-constructed piece of mechan- 

 ism for marking minute portions of time.5 the three hundredth part of 

 a second was indicated by it. It was a kind of watch, with a pin for 

 stopping one of the hands. I proposed that we should each endeavor 

 to stop it twenty times in succession at the same point. We were both 

 equally unpracticed, and our first endeavors showed that we could not 

 be confident of the twentieth part of a sec(fed. In fact, both the time 

 occupied in causing the extremities of the fingers to obey the volition, 

 as well as the time employed in compressing the flesh before the fingers 

 acted on the stop, appeared to influence the accuracy of our observa- 

 tions. From some few exjieriments I made I thought I perceived that 

 the rapidity of the transmission of the eft'ects of the will depended on 

 the state of fatigue or health of the body. If any one were tO make ex- 

 I)eriments on this subject, it might be interesting to compare the rapidity 

 of the transmission of volition in different persons with the time occu- 

 pied in obliterating an impression made on one of the senses of the 

 same persons. For example, by having a mechanism to make a piece 

 of ignited charcoal revolve with different degrees of velocity, some 

 persons will perceive a continuous circle of light before others, whose 

 retina does not retain so long impressions that are made upon it. 



On the Frauds of Observers. 



Scientific inquiries are more exposed than most others to the inroads 

 of pretenders 5 aud I feel that I shall deserve the thanks of all who 



