200 HYDROGRAPHICAL SURVEYING [chap. xii. 



Caicuia ference being in the ordinary corrections to declination, etc. 



Sun Ob- ^^ want the error of the watch on apparent time to calculate 



serva- what it sho^\■s at apparent noon, the time the sun will be on 

 tions. ,1 • i- 



the meridian. 



An example is given on next page. 



On November 15, 1876, circum-meridian altitudes of the 



were observed with artificial horizon at Maghabiyeh I'*. 



Approximate latitude, 18° 15' north ; longitude, 40° 44' east. 



Barometer, 30- 00 inches ; thermometer, 80°. Mean of observed 



altitudes Sun's Upper Limb, 106° 47' 09-1". 



Local There is a source of error in obtaining an accurate altitude 



Attrac 



tion. which must not be forgotten, especially when the scale of a 



chart depends on a difference of latitude, and that is the 



local attraction due to the irregular disposition of masses of 



land in the vicinity of an observation spot. A mountain 



mass on one side, or deep sea, will cause the local direction 



of gravity to slightly diverge from the vertical. The surface 



of the mercury will not in such a case be truly horizontal, and 



the altitude will be in error. Tliis may often largely account 



for differences between triangulation and observation, and 



when the former is good, it may be in certain cases desirable 



to rest on it rather than the scale by observations. Formulae 



have been drawn up for correction, but they rest largely on 



assumptions of mass which cannot be verified. 



This attraction will also affect determination of time, and 

 therefore longitude. 



In mountainous countries, as near the Alps and in the 

 Caucasus, deflections have been observed to the amount of as 

 much as twenty-nine seconds. On the other hand, deflections 

 have been observed in flat countries. In the vicinity of Moscow 

 within a distance of sixteen miles the plumb-line varies as 

 much as sixteen seconds in such a manner as to indicate a vast 

 deficiency of matter in the underlying strata. But these are 

 exceptional cases. On the north coast of Banffshire the deflec- 

 tion amounts to ten seconds. There is the sea to the north 

 and an undulating country to the south, which, however, to a 

 spectator at the station does not suggest any great disturbance 

 of gravity. 



There are many instances of disturbances of tlu-ee to four 

 seconds. 



