36 
PUTNAM. 
mula,* but the difference is unimportant in view of the 
small flexure actually encountered. The flexure was meas¬ 
ured by mounting a microscope independently of the case 
and observing the movement of a glass scale placed above 
the knife-edge when a weight was applied at the end of a 
cord running over a pulley and attached near the edge, the 
force acting horizontally in the plane of oscillation. Of 
course, endeavor was made to have the conditions as uniform 
as practicable at the various stations. The arc and pressure, 
being controllable, were always nearly the same; favorable 
temperature conditions were obtained by setting up the 
apparatus in basement rooms wherever possible; the chro¬ 
nometers were well protected and not disturbed during the 
course of observations at a station, a hack watch taking 
their place in noting the instant of coincidence; and the 
receiver was mounted on masonry piers, stone foundations, 
or concrete floors, except at one station (Norris Geyser basin), 
w T here it was necessary to use wooden posts. 
In this work methods not before used with short pendu¬ 
lums were adopted, with the result, it is believed, of saving 
labor and increasing accuracy. Heretofore half second pend¬ 
ulums have been swung either in the open air (as in Europe) 
or at about two thirds atmospheric pressure (as in this coun¬ 
try). Under either of these conditions the diminution of 
arc is so rapid that the length of single swings has ordi¬ 
narily been limited to about an hour, and this practically 
requires the constant attendance of the observer while the 
observations continue. With this method a large amount 
of labor (including night work) is required to make the 
*dT — — -, where T is the period of the pendulum, dPis the change 
2^. A 
of period on account of flexure of the support, e is the elasticity or displace¬ 
ment divided by force applied, P is the weight of the pendulum, h is dis¬ 
tance center of gravity to center of suspension, and X is distance center of 
oscillation to center of suspension. See “ Comptes Rendus de la Cinqui- 
eme Conference Geodesique International, 1877, Annexe la, I b,” pa¬ 
pers by C. S. Peirce and Cellerier; also Report Coast and Geodetic Survey 
for 1881, Appendix 14. 
