to the Antarctic Regions. 195 



furnished by the sensitive paper above alluded to, by the exposure 

 of a small portion of it to the sun at noon for a given time, suppose 

 ten seconds, and subsequent comparison with a scale of tints. Paper 

 duly prepared for these purposes will be supplied for the use of 

 the expedition. During solar eclipses such paper ought to be ex- 

 posed at intervals of five minutes. 



The temperature of the soil under the direct influence of the sun 

 as indicated by a thermometer barely covered with dry earth, is an 

 element of importance to the botanist, and may be recommended as 

 an apt accompaniment to actinometric observations. The thermo- 

 meter used should have a scale reading at least to 180° Fahr. 



The height of the line of perpetual snow, by whatever indications 

 marked, should also be ascertained, wherever practicable. 



7. Depth of the Sea. 



Soundings to as great a depth as practicable should be taken 

 wherever opportunities may offer. Great difficulty, however, is well 

 known to exist in the way of procuring any exact result, or indeed 

 any result at all in very deep seas ; and various methods (all ob- 

 jectionable) have been proposed and tried. Could any means be 

 provided to keep out the water from a shell, and at the same time 

 ensure its explosion on striking the bottom, the time elapsed, be- 

 tween casting the shell overboard and hearing the explosion, would 

 indicate the depth with great precision ; nor need we fear that, 

 if the explosion took place, the sound would not be heard, sound 

 being propagated through water with infinitely greater sharpness 

 and clearness than through air. To overcome the enormous ex- 

 ternal pressure, and to enable the charge to burst the shell, it is 

 probable that mere gunpowder might not suffice. Should this be 

 apprehended, a mixture of fulminating mercury with the charge in 

 about equal proportions, would probably efi^ect the object. At least 

 we know, from experience, the vast increase of bursting power which 

 is communicated to powder by such addition. It has also been sug- 

 gested that an echo from the bed of the ocean might be heard, were 

 a shell exploded just beneath the surface (as an echo from the earth 

 is heard in the car of a balloon) ; and attempts, though imperfect 

 ones, have been made to subject this proposal to trial, the reason of 

 the failure of which does not very distinctly appear. The maximum 

 depth of the sea is a geological datum of such value, that a few failures 

 incurred in attempts may very well be tolerated when placed in com- 

 petition with the interest of even partial success. 



8. Atmospherical Phenomena. 



There can be little need to call the attention of navigators to 

 anything relating to winds, storms, lightning, &c. ; yet there are 

 some points to which attention may be expressly drawn, viz. to 

 such distribution and movements of the clouds as indicate the exist- 

 ence at the same time of an upper and an under current of Avind 



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