to the Antarctic Regions. 203 



ivory point, which is carefully brought down to the surface of the 

 mercury in the cistern, and the two are known to be accurately in 

 contact when the actual point and its reflexion appear just to touch 

 one another. The scale is laid off from this point from an authentic 

 standard, at the temperature of 32°. 



2nd. The second adjustment is that of the vernier, in which the 

 upper part of the scale terminates, to the surface of the mercury 

 in the tube. For this, both the back and front edge are made to 

 coincide, and brought down so as to form a tangent to the curve, 

 and just to exclude the light between them at the point of contact. 

 In making both these adjustments, it is desirable that the eye should 

 be assisted by a magnifying glass. Before the observation is made 

 the instrument should be slightly tapped, to free the mercury from 

 any adhesion to the glass; but any violent oscillation should be 

 avoided. 



The portable barometer has only one adjustment, namely, that 

 of the vernier to the upper surface of the mercury in the tube, 

 which adjustment must be effected with the same precaution as in 

 the case of the standard instrument. 



This first reading may be entered in the column prepared for it 

 in the register, and beside it the temperature of the mercury care- 

 fully read off from the thermometer which dips into the cistern. 



As, in the case of the standard barometer, the first measure is taken 

 immediately from the surface of the mercury in the cistern, it re- 

 quires no correction for the different capacities of the tube and cis- 

 tern. Neither does it require any correction for capillary action, as 

 the large diameter of the tube renders this correction inappreciable. 



The portable barometer, however, requires corrections for both 

 these circumstances. For the purpose of the former, the neutral point 

 is marked upon each instrument, or that particular height which, in 

 the construction of the instrument, has been actually measured from 

 the surface of the mercury in the cistern. 



It is obvious that, in almost every case, the mercury will stand either 

 above or below the neutral point : if above, a portion of the mercury 

 must have left the cistern to enter the tube, and consequently must 

 have lowered the surface in the cistern ; if below, a quantity of mer- 

 cury must have left the tube, and, entering the cistern, raised the level 

 of the mercury in it. For the correction of observations for this cir- 

 cumstance, the relation of the capacities of the tube and cistern have 

 been experimentally ascertained, and are marked upon the instru- 

 ment : thus, capacity -^ih, indicates that for every inch of elevation 

 of the mercury in the tube, that in the cistern will be depressed one 

 50th of an inch. Thus, when the mercury in the tube is above the 

 neutral point, the difference between it and the neutral point is to be 

 divided by the capacity, and the quotient being added to the ob- 

 served height, the result will be the corrected height. Or if the 

 mercury at the time of observation should be below the neutral point, 

 the difference of the two is to be divided as before, and the quotient 

 to be subtracted from the observed height. Thus, suppose the ca- 

 pacity to be j\yth, the neutral point 30 inches, and the observed 



