38 
PUTNAM. 
arrangement either could be thrown into the flash circuit 
or on to the chronograph. The advantage in the use of 
two chronometers is in the constant check that the compar¬ 
ison of results furnishes against errors of observation or 
computation, and also in the safeguard against accident to 
either time piece. The comparison of the average periods 
of the three pendulums as derived separately from the two 
chronometers at all the stations shows an average difference 
of only .0000001 second, and the maximum at any station 
is only .0000004 second, thus indicating practically the total 
elimination of errors due to diurnal irregularities of rate. 
By comparing the results for separate swings we have some 
measure of the size of these irregularities, amounting in one 
case to .0000027 second, which corresponds to a relative 
variation of daily rate of 0.5 second. The flexure of the 
case and support was measured at all stations and the periods 
corrected accordingly. 
The rates were derived from observations of about eight 
stars made each favorable evening with a portable transit in 
the meridian, usually mounted on wooden posts well braced. 
These observations, as well as the chronometer comparisons, 
were recorded on a chronograph. At three stations the rates 
were kindly furnished by observatory authorities, and at 
several others fixed observatory instruments were used. The 
rather unusual privileges required for the suitable location 
of the stations in the basements of public or private build¬ 
ings were always courteously granted. The astronomical 
tent was ordinarily set up near by. The geographical posi¬ 
tions of the stations were obtained either by connection with 
known points in the vicinity or by approximate determina¬ 
tions, the latitude by observing a few pairs of stars by Tal- 
cott’s method and the longitude by comparing the chronom¬ 
eter corrections at succeeding stations. The elevations 
adopted are mostly based on the data given in Mr. Gannett’s 
“Dictionary of Altitudes in the United States.” Approxi¬ 
mate connection was always made between the pendulum 
