172 M. Knmond's Imtruction$for the Application of [Sept. 



Observations * for the Measurement of Heights. 



In addition to what has been before said of the instruments, 

 and of the manner of using them in general, the following 

 remarks apply to the observations on mountains. 



The mountain barometer should be of such a construction as 

 to be neither liable to be easily broken, nor to the introduction 

 of air bubbles. It should be easy to try whether the instrument 

 takes the vertical position well, and continues in it ; and I 

 regard as a peculiar merit, such a construction that it rapidly 

 acquires the temperature of the place. The resistance of the 

 mounting to variations of temperature exposes the observation 

 to inaccuracies, causes loss of time, and occasions many errors. 



Observations for the measurement of heights necessarily sup- 

 pose corresponding observations if any exactness be intended ; 

 and the two barometers ought to be perfectly comparable, or the 

 precaution will be in vain. In this case, the mere presumption 

 that they agree is not sufficient ; we must carefully compare the 

 instruments, and if the operation be delicate, and we wish for 

 great precision, it is not enough to have made the comparison 

 beforehand alone, but it is prudent to do so after the operation 

 also ; for the portable barometer may have undergone some 

 derangement in carrying. When the two instruments do not 

 sustain the mercury exactly at the same point, if the difference 

 is not great, and especially if it is such as may be owing to the 

 difference of the diameters of the tubes, there is no reason for 

 suspecting either of them; and we may be content to allow for 

 the difference in the calculation. In the case where either of 

 the two instruments does not sustain the mercury at its absolute 

 height (which cannot fail to happen if they are both of the 

 cistern construction), it will be proper to correct them for the 

 effect of capillarity : for the depression resulting from this cause 

 is sometimes sufficient to introduce a sensible error into the 

 measurements. 



One great difficulty consists in finding a suitable place for the 

 portable barometer : it is necessary to preserve it from rapid 

 changes of temperature ; yet it is almost always exposed to the 

 free air where the temperature is continually varying. The 

 instrument ought to be kept in the shade ; yet it is very often 

 exposed to the sun, which acts very unequally on its different 

 parts, whether by direct or reflected rays. The consequences of 

 such a position are more easy to conceive than to avoid : in the 

 sun, on the one hand, the tube becomes heated ; the cistern 

 again is further heated by the reverberation of the ground : the 

 correctional thermometer indicates a temperature more or less 

 elevated according to the direction in which we turn it : then 



♦ *' Observations Ainbulantes.'* 



