1843.] of a Portable Meridian. 441 



6th. To find apparent Noon, the instrument must remain in the 

 same position until next day, and when the luminous round spot occa- 

 sioned by the sun shining through the aperture L falls on the centre 

 line M N of the radius, the sun is at its meridian, and shews apparent 

 noon. By adding or subtracting the equation of time for the day of 

 the month, the mean time, which a watch or clock ought to keep, will 

 be ascertained. 



7th. To find the altitude it is only necessary to mark the point on 

 the radius where the sun crosses it, and read off the altitude.* 



8 th. To find the latitude, deduct the observed angle from 90°, and 

 add the result to the declination if north. If the declination be south, 

 add it to 90°, and deduct the observed angle. At the equinox, the ob- 

 served angle deducted from 90° gives the latitude. 



9th. In order to lay down a meridian line for survey purposes, fix 

 the sight R on to the rim of the instrument opposite the rod o, as 

 shewn in the figure O P. Look through the two corresponding aper- 

 tures (which are exactly on a level with each other) at a pole erected 

 at some distance in the line of sight. Then move round and look 

 through the sight P in the opposite direction at another pole erected 

 to the south in the line of sight, cut a line on the ground connecting 

 the two poles, and your meridian is complete. 



10th. I have constructed a small instrument of this description, the 

 diameter of which A B is 9J inches, and the height of the pedestal or 

 gnomon 2 inches. It is made entirely of brass, and the degrees on the 

 rim have been marked off with great accuracy by a native mistry. The 

 cost of the materials is not more than two rupees. The labour, how- 

 ever, is considerable* and the man asked sixteen rupees for the whole, 

 including his own remuneration. This, however, is cheap compared 

 with one of Dolland's Universal Dials, 4| inches in diameter, which in 

 Calcutta costs eighty rupees. 



11th. Lest a scientific instrument contrived by an unprofessional in- 

 dividual, should be received with doubt or hesitation, I have annexed to 

 this memorandum extracts from two notes from the professional Sur- 

 veyor of Patna, giving his opinion on the accuracy and utility of the 

 instrument, which I hope will be thought satisfactory. The principle 



* If the moon should pass the meridian at night, its altitude and the time may be 

 ascertained in the same manner. 



