VOL. XXXIX.] PHILOSOPHICAL TRANSACTIONS. 51 



Let the lines am, hr meet bq in e and h. Join cf and gs, meeting in 

 D ; join Da, meeting cm and sl in n and r : then if en and hr be joined, 

 these will be lines given in position, by Lemma 1 . For since the points f, c, d, 

 are in the same right line ; and the intersections of the lines fm, cm, and Fa, 

 Da, run over given lines, the intersection of the legs cm, do will also touch 

 the given one. And, for the like reason, since s, d, g are in the same right 

 line, the intersection of do and sl will also touch the given one. 



Therefore, omitting the poles f, g, there is to be found the curve which the 

 intersection p, of cn, sl, will describe, while by the lines cn, dn, sr, revolv- 

 ing about the poles c, d, s, the concourse of the two cn, dn touches the 

 given line en, and the concourse of the two sr, dn touches the given one hr; 

 and that this is a conic section is plain from the foregoing proposition. 



Some Experiments relating to Electricity. By Mr. Stephen Gray, F. R. S. 



N°439, p. 166. 



Feb. 18, Mr. Gray tried what effect would be produced on several sorts of 

 wood with respect to the luminous part of electricity : the wood was made 

 into rods of the same form with those iron ones mentioned in a former letter 

 on this subject ; the woods were fir, ash and holly ; these being successively 

 disposed on electric bodies, after the same manner as the iron rods had been, 

 the tube being applied to one end, there appeared a light on it, but not with 

 so great a force, nor did the light extend to so great a length ; neither was the 

 form of it conical, but rather cylindrical ; but its extremity seemed to consist 

 of a short fringe of light ; when the light, that was given to the rod by the 

 application of the tube, ceased, on a motion of the hand towards the point 

 of the rod, the light came out again, as has been mentioned of the iron rods ; 

 but when the hand or finger was held near the point of these wooden rods, 

 there was no pricking or pushing of the finger felt, as when the iron rods were 

 used. He had some of these rods made much larger at one end than the 

 other, and now applying his finger to the larger end, there not only appeared 

 a light, but the finger was pushed, especially when the holly rod was used, and 

 the cheek was a little pricked, but the smart was not near so great as when 

 the iron rods were used ; the large end of the rod was pointed with a much 

 greater angle than the smaller one, yet there was very little, if any difference, 

 in the form or size of the light from either end. 



Having procured two pair of lines made of worsted yarn, one of them of 

 a mazareen blue, the other of a scarlet colour ; on the 3d of April, he sus- 

 pended the boy first on the blue lines, and found that all the effects were the 



H 1 



