184 BULLETIN OF THE 



top and sides of hills, but may flourish less below ; vitality di- 

 minishing by exposure to winds. 



Mr. G. K. Gilbert made a communication on 



A PROPOSED NEW LEVELLING INSTRUMENT 



for scientific uses. To the usual combination of spirit level and 

 telescope, he proposed to add a graduated rod, so attached that 

 when the rod was adjusted to verticality the optical axis of the 

 telescope would be horizontal. The rod would extend as far 

 below the telescope as it did above, and the tripod head would 

 be pierced to admit the passage of the rod. In use the instru- 

 ments would be handled in pairs, each being in turn carried for- 

 ward past the station of the other. Each reading would be 

 reciprocal, the height of the higher telescope being read on the 

 upper half of the rod of the lower instrument, and the height of 

 the lower telescope on the lower half of the upper rod. The 

 mean of the reciprocal readings would need no correction for 

 curvature of the earth or for normal refraction, and an inspection 

 of the sum of the reciprocal readings would lead to the detection 

 of errors arising from mal-adjustment, from mistakes, or from 

 unfavorable conditions of atmosphere or ground surface. An 

 important use of the instrument would be for the investigation 

 of the conditions affecting the precision of levelling, so as to- 

 determine the most favorable weather, the most favorable hours, 

 and the most favorable character of ground surface. 



Remarks were made by Messrs. Coffin, Hall, and Dall, the 

 latter objecting to the instability of such an instrument even in 

 light winds. 



135th Meeting. January 19, 18Y8. 



Yiee-President Welling in the Chair. 



Fifty-seven members and visitors present. 



The election of Mr. Arnold Burgess Johnson as a member 

 of the Society was announced. 



