Hydrography^ and the Art of Navigation. 39T 



covered to a certain thickness with white wool, and that of the 

 other with an equal quantity of black wool. These two instru-^ 

 raents, exposed to the sun side by side, will never indicate the 

 same degree ; that with the black covering will mount highest. 

 The question, therefore, will consist in determining if the diffe- 

 rence of the two indications is less at the equator than at Cape 

 Horn, or at any other higher latitude.* 



It will easily be understood that comparative observations of 

 this nature ought to be made at equal altitudes of the sun, and 

 during the most serene weather. Slight differences of altitude, 

 however, will not always impair the accuracy of the observations, 

 if care be taken, under different latitudes, to determine accord- 

 ing to what progression the difference of the two instruments 

 increases from sunrise till mid-day, and diminishes from the lat- 

 ter period till sun-set. Days on which the wind is very high 

 ought to be altogether excluded, whatever be the state of the 

 atmosphere in other respects. 



Another observation somewhat analogous to that of the two 

 thermometers differently covered, will consist in determining the 

 maximum temperature which the sun imparts to a dry soil in 

 equinoctial countries. At Paris, in August 1826, during a se- 

 rene state of the sky, we found that a thermometer lying hori- 

 zontally, and having its bulb covered with one millemeter of 

 very fine vegetable mould, stood at -f 54° (1 29°.2 Fahr.). The 

 same instrument, covered to double that depth with river sand, 

 indicated only -|- 46° (114°.8 Fahr.). 



Experiments to be made on the Radiation of the Sky. — The 

 experiments which we are about to propose ought to give, all 

 other things being equal, the degree of the atmosphere's trans- 

 parenc3^ This transparency may be appreciated in a manner 

 in some sort inverse and not less interesting, by observations on 

 nocturnal radiation, which are likewise recommended to the 

 commander of the Bonite. 



• There are other means still more exact for resolving the problem to 

 which the calorific action of the solar ra^s has given rise ; but these depend on 

 instruments which were not to be found in the hands of our artists at the time 

 of the departure of the Bonite, and therefore are not alluded to in the instruc- 

 tions of the Academy. We will return to the consideration of them on an- 

 ather opportunity. 4 



