NATURAL PHILOSOPHY. 127 



and ancien ekve of the Polyteclinic school made an ingenious remark. 

 He supposed that the needle, instead of being placed in the plane of 

 the circle, should be placed completely outside of it, either on the one 

 or the other side, but always perpendicular to the plane of the circle 

 and by its center ; he foresaw that, in these new conditions, the needle 

 would be in the power of the circular current, and that it would be 

 forced to make a greater deviation from its initial position, (parallel to 

 the circle.) He saw that if the needle avoided by one of its extrem- 

 ities, the magnetic action of the current, it would go before this action 

 on the other extremity ; that what was lost on one side would be 

 more than gained on the other, and consequently the violations of the 

 law of tangents must, compared to the preceding case, be produced 

 in an inverse sense ; he observed that in the conditions where the or- 

 dinary tangent-compass became inert, this was endued with increased 

 alacrity. But, by virtue of the great law of continuity, which regu- 

 lates the phenomena of nature, as well as those connected with pure 

 mathematics, there must exist upon that perpendicular to the centre of 

 the circle, a point where the magnetic needle, avoiding both extremes, 

 must necessarily follow a regular march. 



To test his idea by experiment, M. Gaugain went to M. Fronient, the 

 celebrated instrument maker, and ordered a tangent-compass, whose cir- 

 cle could at will be made to move parallel to itself and be placed at differ- 

 ent distances from the centre of the needle. By means of this arrange- 

 ment, he has been enabled to ascertain what is for every position of the 

 circle the corresponding value of the difference between the real and 

 theoretical deviations, which he sought to annihilate. He attained 

 these results : when the circle is of a small diameter and occupies 

 its ordinary position, or when its center coincides with that of the 

 magnetic needle, the value of the difference is very great, when the 

 deviation itself is somewhat great ; but if the circle is removed from 

 the center of the needle, it is found that the difference corresponding 

 to a determined deviation decreases as the circle is removed from the 

 needle ; when it has reached a certain distance, the difference is mil 

 for all the deviations. Beyond this distance, the difference begins to 

 re-appear with a contrary sign, and its absolute value continues to in- 

 crease with the distance, (at least with quite extensive limits.) M. 

 Gaugain next operated comparatively with two different circles, and 

 he observed for both of them that the true place of the center of the 

 needle is outside of their planes, at a distance equal to one quarter of 

 their radius. The analysis applied to this question by M. Bravis, con- 

 firms the result of this experiment. Proc. French Academy. 



SELF-REGISTERING COMPASS. 



M. Deleuil has presented to the French Academy a new self- 

 registering compass. Its object is to register the changes of direction 

 in a vessel for every three minutes during the twenty-four hours. 

 The marking is made upon a compass card ; and it enables the captain 

 to control with certainty the direction followed by his ship, and to 



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