1843.] of a Portable Meridian. 439 



enable any person, however unscientific, to determine the variation of 

 any needle in a minute by mere inspection. The altitude of the sun 

 or moon,* when on the meridian, can be read off with equal ease, and 

 the latitude of any place ascertained with the aid of a Table of Decli- 

 nation pasted on the lid of the box. By fixing sights at each end of 

 the meridian line, the instrumeut would serve for taking levels ; and 

 last, though not least, would enable Surveyors to lay down a long 

 meridian line for the base of all their triangles, with much less difficul- 

 ty than is experienced in many of the usual methods. Though simple 

 and easy in theory, many of these methods are difficult in practice. 

 They require that an officer, perhaps suddenly ordered out to make 

 the survey of a district, should be in the possession of certain instru- 

 ments and certain astronomical works, which are not always to be ob- 

 tained. For instance, the most approved method of laying down a me- 

 ridian is said to be by observations of equal altitudes of the Polestar ; 

 but without the Nautical Almanac for the year, which is not always 

 to be obtained, it often requires nights of watching, and the patience 

 of a Chaldean to catch the star in the small field of a Theodolite teles- 

 cope at the precise moment necessary for the accuracy of the observa- 

 tion. The process by observation of the sun's azimuth is also I under- 

 stand not free from difficulties. Under these circumstances it is hoped, 

 that the simple instrument now submitted for consideration, (though it 

 does not pretend to perfect accuracy,) may be occasionally found use- 

 ful by the scientific as well as by the unscientific world. With these few 

 explanatory remarks, I proceed to describe the instrument, a sketch 

 of which accompanies this memorandum. 



4th. A B C is a brass semicircular plate, about 2-10ths of an inch 

 thick, with the degrees marked on the rim, which are counted from the 

 point Q. both to the right and left, D C being of course at right an- 

 gles to A B. E F is a moveable radius turning on the point E, and hav- 

 ing degrees of altitude marked on it, as shewn in M N. The mode of 

 laying down the degrees by means of a graduated circle will be under- 

 stood from the figure S T V ; K L is a perpendicular flat rod having a 

 small oblong plate L with a hole in the centre, fixed at right angles to K 

 and parallel with the horizon. This rod KL is to be fixed at Df perpen- 



* At night. 



f As exemplified in the figure P. 



