NATURAL PHILOSOPHY. 173 



traceable in the course of rivers, and their frequent pressure upon their 

 right banks. 



ON THE POSSIBILITY OF STUDYING THE EARTH'S INTERNAL STRUC- 

 TURE FROM PHENOMENA OBSERVED AT ITS SURFACE. 



Professor Hennessy, in a paper on the above subject, read before the 

 British Association, I860, considered the possibility of obtaining results from 

 the comparison of the level surface, usually called the earth's surface by 

 astronomers and mathematicians, with the geological surface which would 

 be presented if the earth were stripped of its fluid coating. At present the 

 number of unknown quantities in an inquiry as to the earth's internal struc- 

 ture was greater than the number of conditions; but, by knowing the true 

 surface, and adopting the results of established physical and hydrostatical 

 laws relative to the supposed internal fluid mass, we should be able to estab- 

 lish as many equations as we have unknown quantities, and thus obtain a 

 solution. 



Professor Stevely stated, that the exact spheroidal form of the earth, and 

 the direction of gravity at each part of its surface, were not so completely 

 determined as the remarks of Professor Hennessy would lead a person to 

 suppose. Very interesting papers, printed in the last volume of the Trans- 

 actions of the Royal Society, by Colonel Sir Henry James and Captain Clarke, 

 had shown conclusively that not only did the spheroidal form of the earth, 

 as deduced from the great Ordnance Survey of the British Islands, differ 

 somewhat from that considered as most suitable to the form of the earth, as 

 derived from a comparison of all observations; but even particular localities 

 had the plumb-line so affected by local circumstances that the forms, as 

 deduced from particular portions of the survey, differed sensibly from one 

 another. Thus, the plumb-line near Edinburgh was found to be affected 

 not only by the proximity to Arthur's Seat and the Calton Hill, but even 

 the defect of matter in the Frith of Forth, and the excess in the distant Port- 

 land Hills, were shown to exercise important influences. 



Colonel Sir H. James showed, by various examples, that the method of 

 grouping the measurements of different countries, proposed by Mr. Hennessy, 

 would not, in the present state of these measurements, lead to the exact 

 results he supposed. He then pointed out circumstances not only respecting 

 the Russian measurements, but even the French, which would make a 

 reexamination of them not only desirable but necessary. 



NEW METHODS OF DETERMINING SPECIFIC GRAVITY. 



Until a few years ago the determination of the specific gravity of solids 

 was conducted on one of these two principles : either by finding the loss of 

 weight a body suffered on being immersed in water (its absolute weight being 

 known), with the aid of the hydrostatic balance or Nicholson's areometer, 

 or by determining, by means of a balance, the quantity of liquid displaced 

 by the substance in question ; the vessel used being a little flask holding a 

 certain weight of water (one thousand grains). 



Since the general introduction of the volumetric assay, and with it of 

 graduated cylinders, the thought lay very near, in determining the specific 

 gravity of dry substances, to measure instead of weighing the quantity of 

 liquid displaced. F. Mohr has demonstrated that by far the simplest plan is 



