VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 389 



to do, by putting the arm in motion, and observing the time of its vibrations, 

 from which it may easily be computed.* 



Mr. Michell had prepared 1 wooden stands, on which the leaden weights were 

 to be supported, and pushed forwards, till they came almost in contact with the 

 case; but he seems to have intended to move them by hand. As the force with 

 which the balls are attracted by these weights is excessively minute, not more than 

 , , 6 > „ 6 of their weight, it is plain that a very minute disturbing force will be 

 sufficient to destroy the success of the experiment; and from the following experi- 

 ments it will appear, that the disturbing force most difficult to guard against, is 

 that arising from the variations of heat and cold ; for if one side of the case be 

 warmer than the other, the air in contact with it will be rarefied, and in conse- 

 quence will ascend, while that on the other side will descend, and produce a cur- 

 rent which will draw the arm sensibly aside.-)- As I was convinced of the necessity 

 of guarding against this source of error, I resolved to place the apparatus in a 

 room which should remain constantly shut, and to observe the motion of the arm 

 from without, by means of a telescope; and to suspend the leaden weights in such 

 manner, that I could move them without entering into the room. This difference 

 in the manner of observing, rendered it necessary to make some alteration in Mr. 

 Michell's apparatus; and as there were some parts of it which I thought not so 

 convenient as could be wished, I chose to make the greatest part of it anew. 



Fig. 1, pi. 7, is a longitudinal vertical section through the instrument, and the 

 building in which it is placed : abcddcbaeffe is the case ; x and x are the two 

 balls, which are suspended by the wires hx from the arm ghmh, which is itself 

 suspended by the slender wire gl. This arm consists of a slender deal rod hmh, 

 strengthened by a silver wire hgh ; by which means it is made strong enough to 

 support the balls, though very light. f The case is supported, and set horizontal, 

 by 4 screws, resting on posts fixed firmly into the ground : 2 of them are repre- 

 sented in the figure, by s and s ; the other 2 are not represented, to avoid confusion. 



* Mr. Coulomb has, in a variety of cases, used a contrivance of this kind for trying small attrac- 

 tions ; but Mr. Michell informed me of his intention of making this experiment, and of the method 

 he intended to use, before the publication of any of Mr. Coulomb's experiments. — Orig. 



-j- M. Cassini, in observing the variation compass placed by him in the observatory, (which was con-. 

 structed so as to make very minute changes of position visible, and in which the needle was suspended 

 by a silk thread), found that standing near the box, in order to observe, drew the needle sensibly 

 aside ; which I have no doubt was caused by this current of air. It must be observed,- that his com- 

 pass-box was of metal, which transmits heat faster than wood, and also was many inches deep ; both 

 which causes served to increase the current of air. To diminish the effect of this current, it is by all 

 means advisable to make the box, in which the needle plays, not much deeper than is necessary to pre- 

 vent the needle from striking against the top and bottom. — Orig. 



J Mr. Michell's rod was entirely of wood, and was much stronger and stiffer than this, though not 

 much heavier ; but, as it had warped when it came to me, I chose to make another, and preferred this 

 form, partly as being easier to construct and meeting with less resistance from the air, and partly because, 

 from its being of a less complicated form, I could more easily compute how much it was attracted by 

 the weights. — Orig. 



