TERRESTRIAL MAGNETIC INTENSITY. 3 



piece of iron or steel (usually a penknife), and the process repeated until the semi- 

 arc of vibration exceeded 20°, if 300 vibrations were to be observed, or 10° if 100 

 only were observed, as was more usually the case. This last deviation from Pro- 

 fessor Hansteen's practice was not adopted without due consideration. The large 

 commencing arc necessary in order that the vibrations might be distinguishable 

 at the close of 300, increased greatly the errors pointed out by Mr Harris. More- 

 over, there seemed less chance of error in combining several series of 100 vibra- 

 tions taken in succession, than in using a single series of 300, which, from the 

 time it occupies, is more liable to be interrupted and rendered useless by a gust 

 of wind, or a momentary relaxation of the painful attention required to be exerted 

 by an unassisted observer. Besides, the mere error of the observed time, depend- 

 ing on the eye and ear of the observer, will not exceed even in 100 vibrations the 

 uncertainty arising from causes impossible to eliminate, — indeed falls much short 

 of it : yet this is the only error which we diminish by increasing the vibrations in 

 a series to 300. 



8. When the semi-arc of vibration had diminished to 20°, or to 10° (as 300, 

 or 100 vibrations were to be observed), the counting of vibrations commenced, — 

 the hour, minute, second, and decimal, of the beginning or 0th vibration being 

 noted, and the second and decimal only for each succeeding 10th vibration, until 

 360 vibrations (in the first case) or 160 (in the second) were observed. These se- 

 conds of time are arranged in columns, so that the times of the 0th, 100th, 200th, 

 300th vibration, run along the same horizontal line as do the 10th, 110th, 210th, 

 310th, &c. The time of the 0th being then subtracted from the time of the 300th 

 (or 100th), we have one value of the time of 300 (or 100) vibrations. The 10th, 

 from the SlOtli (or the 110th) gives a second value, and so on to the 60th and 

 360th (or 160th), which gives in all seven values of time of 300 (or of 100) vibra- 

 tions ; the mean of which seven values is taken (the minutes being of course sup- 

 plied), and the hour, minute, and second of conclusion. The thermometer (en- 

 closed in the box) is consulted at the beginning and end, and its indications re- 

 gistered. The rate of diminution of the semi-arc of vibration is ialso observed, its 

 continued bisection being indicated opposite to the instant at which it occurs in 

 a column parallel to those already named. The rate of the chronometer is like- 

 wise to be determined. An example wiU best illustrate all this. 



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