204? Instructions for the Scientific Expedition 



height 30*500 inches, the difference is 0*5 inch, which divided by 

 50 gives 0*01 inch to be added to the observed height, producing 

 30*51, the corrected height; or if the observed height be 29 inches, 

 the difference, 1 inch, divided by 50, gives '02 inch to be sub- 

 tracted from the observed height, giving 28*980 inches for the cor- 

 rected height. 



The second correction required is for the capillary action of the 

 tube, the effect of which is constantly to depress the mercury in the 

 tube by a certain quantity inversely proportioned to the diameter of 

 the tube. In the instruments furnislied to the fixed observatories 

 the amount has been experimentally determined during their con- 

 struction, and marked upon the instrument ; the quantity is always 

 to be added to the height of the mercurial column, previously cor- 

 rected as before. For the convenience of those who may have ba- 

 rometers, the capillary action of which has not been so determined, 

 a table of the corrections for tubes of different diameters is placed 

 in the appendix. 



The marine barometers furnished to the Antarctic Expedition differ 

 in nothing from the other portable barometers but in the mode of 

 their suspension, and the necessary contraction of the tubes to pre- 

 vent oscillation from the motion of the ship, and require the same 

 corrections. 



When these two corrections have been made in the first reading 

 of the portable barometer, it should agree Avith the direct observa- 

 tion of the standard barometer ; and it is very desirable that frequent 

 comparative observations should be made of the two instruments, in 

 order to ascertain whether there may be any permanent difference 

 between them. Should this be the case, the amount may be marked 

 upon the instrument, and allowed for as an index error, in order that, 

 if an accident should happen to one, the other may be substituted 

 for it without detriment to the regular series of observations. 



It is to lie presumed that the portable barometer will frequently 

 be employed in ascertaining tbe altitude of remarkable points in the 

 vicinity of the observatories, or of the more permanent stations of 

 the Expedition. 



The instruments furnished to the observatories have been all in- 

 dependently graduated and compared with the standard of the Royal 

 Society ; and in all cases it is desirable that such a comparison should 

 be made with some standard instrument of authority, directly, or by 

 means of a good portable barometer. In making such comparisons, 

 all that is necessary is to record five or ten simultaneous readings of 

 both instruments, deliberately made, at intervals of a few minutes from 

 each other, after at least an hour's quiet exposure, side by side, that 

 they may have the same temperature. If compared by two observers, 

 each should read off his own barometer in his usual manner, then 

 each should verify the other's result. By this means the zero of one 

 standard may be transported over all the world, and that of others 

 compared with it ascertained. To do so, however, with perfect effect 

 requires the utmost care in the transport of the intermediate baro- 

 meter, and is by no means an operation either of trifling import or 



