ADMIRALTY INSTRUCTIONS. 



23 



at the surface also ; and this last ought to be again 

 observed at the corresponding hour of the night. 



"3. A collection of good observations, systemati- 

 cally continued, for the purpose of connecting the 

 isothermal lines of the globe, and made, as above, 

 at certain uniform depths. 



" 4. Some very interesting facts might result from 

 the comparison of the direct heat of the solar rays 

 in high and low latitudes. The two thermometers 

 for this purpose should be precisely similar in every 

 respect ; the ball of the one should be covered with 

 white kerseymere, and of the other with black ker- 

 seymere, and they should be suspended far out of the 

 reach of any reflected heat from the ship, and 

 also at the same elevation above the surface of the 

 water ; the observations should be made out of 

 sight of land, in a variety of latitudes, and at diffe- 

 rent hours of the day, and every pains taken to 

 render them all strictly similar and comparative. 



" 5. All your meteorologic instruments should be 

 carefully compared throughout a large extent of 

 the scales, and tabulated for the purpose of apply- 

 ing the requisite corrections when necessary, and 

 one or more of them should be compared with the 

 standard instruments at the Royal Society or Royal 

 Observatory on your return home. 



"6. All observations which involve the compari- 

 son of minute differences should be the mean result 

 of at least three readings, and should be as much 

 as possible the province of the same individual 

 observer. 



