172 



dional hours, when the daily variation is most rapid, this may be fol- 

 lowed almost from one minute of time to the other. 



Of equal importance is this mode of proceeding for observing the 

 duration of the vibrations. The passage of the vertical thread in the 

 telescope before a fixed point of the scale (properly speaking, the re- 

 verse is the case), may, even if the whole deviation only amount to 

 a few minutes, be observed with such a degree of precision as never to 

 leave any uncertainty amounting to the tenth of a second in time. The 

 considerable duration of a vibration (about 14 seconds in the most in- 

 tensely magnetized needles), and the slow degrees by which the arc 

 decreases, are productive of other important advantages : only a few 

 vibrations are required to enable us to determine the time of one vi- 

 bration with such accuracy, that, though the needle be left to itself 

 for one or even several hours, no doubt will remain on the mind of 

 the observer as to the number of oscillations performed during the 

 interval of his absence. We may commence with vibrations so small 

 (such, for instance, as those with which we generally leave of}',) that 

 the reduction to infinitely small vibrations becomes almost impercep- 

 tible ; and yet, after an interval of six and more hours, the vibrations 

 are still found sufficiently great to admit of having their beginnings 

 observed with all requisite precision. 



In cases where anomalies still appear in the observations (which, 

 however will prove so trifling, that with the common means they 

 would have been altogether imperceptible,) they are solely to be 

 ascribed to the current of air which, in the locality where the experi- 

 ments were made, could not be altogether avoided. To remedy this 

 inconvenience the aperture of the box might be closed by a plane 

 glass j but none perfectly true was within the author's reach, neither 

 could it have been made use of without an inconvenient loss of light. 



To the enumerated advantages of this method another may be 

 added, which is, that the observer constantly remains at a great di- 

 stance from the needle, while in the old mode of proceeding his 

 proximity to it was unavoidable j so that, even if enclosed in a glazed 

 case, it was exposed to the disturbing influence which might be ex- 

 erted upon it by the warmth of the body, or that of the lamp, by the 

 iron or even the brass which the experimenter might happen to carry 

 about him. 



The advantages of stout heavy needles over those of diminutive 

 size, which have been made use of for most magnetical observations, 

 particularly those relating to the time of vibration, are dwelt upon 

 by Professor Gauss ; he has since successfully employed one weigh- 

 ing upwards of two pounds, and expresses his conviction, that if needles 

 of from four to six pounds in weight were used, on which slight cur- 

 rents of air would cease to exert any perceptible influence, magnetic 

 observations might attain an exactness and precision unsurpassed by 

 the most delicate astronomical observations. Much stronger threads 

 would indeed be required tor suspension, the torsion of which would 

 produce greater reaction ; but whatever the strength of the thread 

 may be, the force of torsion must always, and may without any diffi- 

 culty, be taken into account with the greatest exactness. 



