Professor Leslie (yn the Theory irf Comjn'cssion. 85 



the internal structure of our globe, which I purpose soon to 

 expand and illustrate. In the mean time, I shall offer a few 

 preliminary remarks. 



Though we must not push the law of compression to its ut- 

 most extent, we may safely infer that it stretches far beyond 

 the limits of ordinary experiments. Air has been condensed 

 120 times, without betraying any sensible deviation from the 

 principle, that its elasticity is proportionally increased. Water 

 was rendered l-36th part denser under a powerful compressing 

 engine by Abich and Zimmerman in 1779- Nor can it be 

 doubted, that, with a higher pressure, and more elaborate ma- 

 chinery, much greater changes could be effected in the consti- 

 tution of these fluids 



We may hence fairly conclude, that our planet is not com- 

 posed of an accumulate mass of such materials as are found 

 near its surface, but is very widely cavernous. But how can 

 this vault be sustained, and what rare medium can exert such a 

 vast repulsive energy as to poise the enormous compression of 

 columns 3500 miles in altitude.? Light alone seems to possess the 

 character of extreme rarity, united with elasticity of transcending 

 force. It is known in two very different states — in rapid emis- 

 sion, or in latent combination with other substances. But may 

 not this subtle fluid exist as a distinct collected body, its repul- 

 sive efforts being restrained either by compression or the mu- 

 ' tual attraction of its remote particles ? 



It is an important consideration, that light must be emitted 

 from every substance with the same celerity, for it otherwise 

 would not be fit to serve the purposes of vision. By com- 

 paring that extreme rapidity with the rale with which air would 

 rush into a vacuum^ we are enabled to compute its propulsive 

 power, and contrast its prodigious elasticity. 



*' This capital experiment in the text was first devised and per- 

 formed on a small scale by Mr Canton in 1760. It established 

 incontestibly the compression of water, but seems to have been 

 generally overlooked by succeeding popular writers, many of 

 whom still continue to repeat the erroneous conclusion of the 

 Academicians del Cimento, which represents that fluid as abso- 

 lutely incompressible. The instrument alluded to holds about 

 12 lb of water, which was introduced with greajt care and pa- 



