LEVELING, AliTD USE OF BAEOMETEE. 71 



The object sought viz: The difference of altitude of objects 

 may be attained in various ways depending upon the degree of ac- 

 curacy required and the instruments at hand. 



The masons level, the builders level, and the surve^^ors level all 

 serve their purpose in their respective places. The construction 

 and value of these instruments varies with their real worth as com- 

 pared with their requirements. 



The Theodolite and Sextant are sometimes used in leveling, in 

 such case it is nothing more or less than a vertical triangulation 

 and is treated and computed as such. 



in connection with the sextant there is used an artificial horizon. 

 This is simply a reflecting liquid by means of which the direct and 

 reflected images of an object are brought in contact in the horizon 

 glass of the sextant and the angular distance measured, the angle 

 as a matter of course is double the angle of elevation above the ap- 

 parant horizon. Mercury is most generall}^ used, but oxidises rap- 

 idly when in contact with air, and although a very dense substance 

 is most easily disturbed by the slightes breeze. Oil, colored with 

 lamp-black or molasses, are about as convenient and reliable as any- 

 thing for the purpose, and being inexpensive can be renewed when- 

 ever desirable. 



Ln measuring great differences of altitude recourse is had to 

 the barometer. It is not reliable for small differences as its motion 

 is but one-tenth of an inch for altitudes of from 96 to 110 feet and 

 is moreover affected by every change of temperature and the con- 

 sequent change of densitj^ of the atmosphere. 



Operations with the barometer are based upon the principle of 

 the Torricellian vacuum which is simply a measure of the weight 

 of the atmosphere. If now according to Harriot's law "the density 

 of one and the same quantity of air is proportional to its tension." 

 we have at once a means of measuring the difference of heights by 

 the tension of the atmosphere, for as we ascend, the density de- 

 creases as the column. 



This motion as before stated is so gradual (one inch for 1,000 

 feet) and affected by so many contingencies that the barometer has 

 always, and now is, looked upon as a very uncertain and unreliable 

 leveling instrument. 



This, however, will vanish with experience, and with proper care 

 and application the barometer will be found very useful and quite 



