OF ARTS AND SCIENCES. 293 



time, it might be better to have a series of lines in the field, and ob- 

 serve the transits over each, as in a meridian instrument. The advan- 

 tages of the two forms of instrument employed, attaching the level to 

 the telescope or to the wyes, will vary with the surrounding conditions. 

 The principal objection to the second method is the time required to 

 determine the constants. Tiiis may be done almost equally well when 

 the level is attached directly to the telescope, by taking reciprocal read- 

 ings from two points one or two hundred yards apart. The variations 

 of the instrumental constants will also doubtless be less with this form 

 of instrument. In either case, if many observations are to be taken 

 from a given station, it is advisable to determine once for all the abso- 

 lute altitude of some convenient object, and refer everything to that, 

 like the meridian mark of a transit iiistrument. 



