584 KEPORT— 1888. 



The tension was ascertained from the enlargement due to the pull of the liquid 

 on the ellipsoidal bulb of a thermometer sealed into the containing vessel. 



By means of this instrument (exhibited), for the construction of which the 

 author had to thank his friend Mr. Charles F. Casella, he had already in a first 

 trial succeeded in subjecting alcohol to a pull of fifteen atmospheres, or 223 lbs. to- 

 tbe square inch, with an extension tliat appeared to be about ~ of the whole 

 volume, or about three times what would have been expected bad the coefficient of 

 extensiljility been the same as that of compressibility. This result, however, which 

 depends on a single measure, when the apparatus, unfortunately, broke under the 

 great strain upon it, requires further confirmation. 



Measures of this kind afford an experimental determination of points in the 

 unstable portion of the isothermal curve of a substance passing from liquid into 

 vapour, which portion lies, as the author pointed out in a paper on ' The Surface 

 Forces in Fluids,' ' below the line of zero pressure, and not as indicated in Andrews' 

 diagram, reproduced in Maxwell's '' Heat.' 



The experimental proof that tensional stress within a mass of liquid is neces- 

 sarily accompanied by a corresponding strain, is, in the author's opinion, an im- 

 portant point in the theory of surface tension, since it shows that the diminution 

 of density or extension of the surface liquid, which can be shown to be a necessity 

 of the equiUbrium at the surface, is sufficient to account for the surface being a 

 seat of energy. It becomes, in fact, nnnecessaiy to ascribe to the energy any other 

 form than that ia which it exists in stretched matter. 



5. A new Sphere Planimeter? By Professor Hele Shaw, M.Inst.G.E. 



The want has long been felt of some instrument for measuring areas which, 

 while possessing the accuracy of tbe Amsler planimeter, would have the great 

 advantage of giving a reading by means of a pointer moving over a dial face of 

 such magnitude as to obviate the use of a vernier. The author, four years ago, 

 brought forward a class of sphere intefcrator, under the name of sphere and roller 

 mechanism, in which, by employing the rolling of two surfaces in contact with 

 each other, instead of the combined slipping and rolling of the Amsler type of 

 instrument, a satisfactory solution of the problem appeared to have been attained. 

 Various forms of these integrators were thoroughly tested, but with unsatisfactory 

 results, inasmuch as they always gave a slight eiTor of variable amount, and it has 

 since been found by means of specially designed experiments that the universally 

 accepted principle of rolling contact relied upon in their design did not hold in 

 practice under the particular conditions in which it was there applied. The sphere 

 planimeter which was exhibited for the first time really belongs to the Amsler 

 class of integrator, though resembling in external appearance the author's previous 

 instrument above referred to, since one essential feature of it is a sphere. 



The instrument consists of a bent bar, one end of which forms the fixed centre, 

 upon the surface containing the area to be measured. The other end is jointed to 

 a frame which guides a sphere by means of four rollers, the centres of these rollers 

 being carried upon small rigid brackets. The frame which supports the dial is 

 continued by means of a bar, at the end of which is a pointer which is passed round 

 the perimeter of the figure to be integrated. Upon the spheres rests a small 

 measuring roller or integrating wheel, the axis of which carries the recording index 

 and a pinion. 



This pinion gears with a wheel ten times as large, working on the back of the 

 dial, by means of which higher readings are recorded. Undue pressure between 

 the measuring roller and the sphere is prevented by means of the roller which is 

 attached to the bar. 



The proof of the theory of the action of the instrument was then given, and the 

 author concluded by pointing out that the action of the sphere is simply to transmit 

 the motion of the roller, and therefore so long as no slipping takes place on the 

 surface of the paper beneath the record is given with the same degree of accuracy 



' Phil. Mnff, = Published in extenso, Engineering, Sept. 21. 



