82 BULLETIN OF THE 



that in a vacuum chamber, above a certain size, the balance disk- 

 arms should not revolve without increasing the rarefaction. 



Mr. Taylor replied that he so understood Mr. Stoney's expo- 

 sition, though he had neglected to state it fully. 



Mr. F. P, JuDD made a communication on 



the ADIRONDACK WATER-SHED, 



referring particularly to the transformation of the lakes of that 

 region into reservoirs, thus checking the spring freshets in the 

 Hudson River and its tributaries, and providing a supply of water 

 in the season of droughts. 



Remarks were made by Mr. Hilgard, by Mr. Holden in refer- 

 ence to a Report made by Maj. Farquhar, of the U. S. Engi- 

 neers, on similar measures for the Mississippi River, and by Mr. 

 Abbe on the constructions for the same purpose in the Rhine 

 and other European rivers. 



Mr. F. V. Greene read a paper on 



THE DEVIATIONS OP THE PLUMB-LINE AS DETERMINED IN THE 

 SURVEY OF THE 49TH PARALLEL OF LATITUDE. 



(^This paper is published in fnll in No. X.L. of the Printed Papers of the 

 Essayons Club of the Corps of Engineers.") 



(abstract.) 



The object of this paper was to investigate the causes of the 

 discrepancies between the astronomical and geodetic determina- 

 tion of points on the 49th parallel of latitude; and it was illus- 

 trated by maps and diagrams showing the topography and geology, 

 and the deviation at each station. 



This parallel (the International Boundary line) was determined 

 and marked for 853 32 miles, from the Lake of tne Woods to the 

 Rocky Mountains : the basis of the determination rested on 47 

 astronomical stations, at each of which the latitude was observed 

 with the Zenith Telescope. Adjacent astronomical stations were 

 connected, and intermediate points of the parallel determined, 

 by the method of "tangents and offsets." These operations were 

 explained sufficiently in detail to show that the greatest uncer- 

 tainty in the astronomical and geodetical determinations at any 

 one point could not exceed 40 feet, or 0".4. Yet the observed 

 discrepancies had an average value of 2". 15, a maximum of 7". 28 



