380 Prof. Sylvester on the Analogues in Space 



the weighings is O011 per cent.; with the second, 0*007 per 

 cent.; and with the third, 007 per cent. 



(3) Dr. Siemens says that we ought to have inverted the tubes 

 in the bridge, and taken the readings in both directions. We 

 did not do this, but we did what amounted to the same ; we took 

 the zero-point of the scale immediately after each series of read- 

 ings. It must be remembered that our bridge was so arranged 

 that a millimetre movement of block corresponds to a change in 

 the ratio of the resistances of about 0*01 per cent., whereas in 

 Dr. Siemens' s bridge it corresponds to 04 per cent.; also that 

 we used as bridge-wire a platinum-iridium alloy (2 millims. dia- 

 meter), which on heating varies in conducting-power much less 

 than platinum, of which Dr. Siemens' s bridge-wire (0°16 millim. 

 diameter) was made. 



One point which neither Dr. Siemens nor Mr. Sabine mention 

 in their papers is, whether their weights and metre-scales were 

 normally accurate ; I mean whether they took the precaution to 

 ascertain, as we did with ours, that their gramme and metre were 

 an absolute gramme and metre. I do not for a moment doubt 

 that their grammes, or the divisions of their scales, agreed well 

 with one another ; in fact they must have done so, otherwise the 

 results obtained could not agree so well with each other. Also 

 they do not state whether they used the method of double weigh- 

 ing or not. All our weighings were made in this manner. 



I must leave it to others to judge whether the experiments 

 made by Mr. Hockin and myself with mercury have not been 

 carried out with as great a care and with as many precautions 

 to ensure correct results as those by Dr. Siemens or Mr. Sabine; 

 and in conclusion I would point out that as the metrical system 

 is gaining yearly more and more ground, in science as well as 

 in every-day life, chiefly owing to its forming a coherent system 

 of measurements (length, weight, capacity, surface), so also will 

 doubtless the B. A. unit, forming a part of a coherent system of 

 electrical measurements (resistance, current, quantity, electro- 

 motive force), gradually gain ground and supplant all others. 



LVII. Supplemental Note on the Analogues in Space to the Car- 

 tesian Ovals in piano. By Professor Sylvester, LL.D., 

 F.R.S.* 



TO complete the theory given in the last Number of the 

 Magazine, concerning the new ovals in space, I ought to 

 notice that the focal cubic there spoken of is the circular cubic 

 of which the four points in the circle, by means of which it is 

 determined, are the foci. This will become evident from com- 

 * Communicated by the Author. 



