MR EDWARD SANG ON THE DEFLECTION OF THE PLUMMET. 93 



summer and the winter solstice. On account of the deflection, our summer 

 observations give -"0050 too little, and our winter ones '0012 also too little, thus 

 giving the obliquity -0019 too small. Simultaneous observations are carried on 

 at Madras, in Lat. 13° 04' 10" north. At the summer solstice the sun is north of 

 the zenith, and its altitude, counting from the south point of the horizon, is -0019 

 too much, while the altitude at the winter solstice is too little by -0047, thus 

 giving the obliquity too great by -"OOSS. Hence observations made at St Peters- 

 burg and at Madras must differ from each other to the extent of '0052, in deter- 

 mining this primary astronomical quantity. 



Or if in the year 1857, we had sought to determine the greatest inclination of 

 the moon's orbit, the greatest meridian altitude must have been -0145, and the 

 least -0009, each too little, as seen from St Petersburg, giving the obliquity 

 '0068 too small; while, as seen from Madras, the upper passage would have 

 appeared -0082 too far north, and the lower -0164 too far south, making an error 

 in excess of -0124, and thus the discrepancy of the two determinations would 

 have amounted to "0192, or about the fiftieth part of a second. 



From these examples it appears, that we cannot carry our astronomical re- 

 searches safely to the hundredth parts of seconds, without having taken into 

 account the displacement of the nadir point. It is vain to seek to eliminate this 

 source of error by the multiplicity of observations, because the influence varies 

 periodically with the hour of the day, the moon's age, the position of the node, 

 and even that of the apogee. 



VOL. XXIII. PART I. 2 C 



