Terrestrial Magnetic Intensity. 



61 



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, se- 

 cond, 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 seconds 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 310th (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) vibrations ; 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 (inclosed in the box) is consulted at the begin- 

 ning and end, and its indications registered. The rate of di- 

 minution of the semi-arc of vibration is also 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 likewise to be determined. 

 An example will best illustrate all this. 



