Miscellanies. 1 9 1 



5. Rock crystals substituted for crown glass in making telescopes . 

 I — ^The celebrated artist Cauchoix, has iniformed Dr. B. Lyiule Oli- 



ver, that by employing rock crystals instead of crowni glass he can 

 make a reduction of one third in tlie leugtli of his telescopes, uiih 

 the farther ab vantage of an increase in the amplifying power and 

 light of one eighth j but this improvement he states, is limited to tel- 

 escopes whose aperture does riot exceed five Inches. — Letter of Dr. 

 Oliver to the editor, dated Dec. 27, 1829- 



1 



f 

 t 



6. On the Temperature of the Sea. (Communicated to die Ec'ilor.) 



L By Paul Swift, M- D. 



Nantucket, February IClh, 1S30. 



Professor Silliman — ^In the last No. of the American Journal of 

 Science and Arts, there is an account of several interesting experi- 

 ments, made by different navigators, showing the temperature of the 

 ocean at great depths.* 



Thy correspondent says, " It seems to me tliat die facts detailed 

 by M. Peron are inconsistent with the theory of M. Cordier, as to 

 central heat, if they do not prove conclusively its entire fallacy." 



Now it seems to me, that neither the experiments of Peron, nor 

 those of any other navigator adduced, justify, or in any degree coun- 

 tenance this conclusion. 



All the experiments reported agree in establishing the facts, that 

 the temperature of the surface of die water varies with tliat of the 

 superincumbent air, and differs but little from it, and diat at greater 

 depdis, the water preserves much more nearly a uniform tempera- 

 ture. 



From these facts it will necessarily follow, that die comparative 

 heat of water at the surftice, and at great depths below it, will de- 

 pend on the temperature of the air at the time and place of making 

 the experiment; thus, if the experiment be made, as by Peron, 

 within the tropics and near die equator, the surface never being less 

 than 86° of Fahrenheit, the mercury will fall in proportion to the 

 descent of the instrument in the water ; if, on the contrary, the ex- 

 periment be made in a high latitude, when the temperature of the 

 air or surface is low, as twice occurred in those made by Forster, 



Communicated by B. Tappan, Esq. of Steubenvinc,Ohio. 



