Dr. Schroder van der Kolk on the Velocity of Sound. 43 



previously been taken into account, he obtained the following 

 results from the four combinations of triangles previously calcu- 

 lated by Moll :— 



17669-510 



17669-498 



17669-510 



17669-518 



Mean . 17669*509 metres. 



The direct determinations of time were made by means of 

 clocks with conical pendulums indicating thirds. During the 

 observations these were constantly compared with two chrono- 

 meters, one of which was at each station. 



Eighty-nine comparisons at the Zevenboomen station are 

 recorded, and hence the value of one minute of mean time is 

 calculated as =69*63 revolutions of the pointer of the thirds- 

 clock. The probable error of this mean was found to be 0*0036 



0*0036 1 ' _ . 



° r "69^63 = 23000* 0fltsvalue ' 



Twenty-one comparisons at the Kooltjesberg station are 

 recorded, according to which one minute was equivalent to 

 69*433 revolutions, with a probable error of 0*023 or 3 4^0 t- 



The rate of the chronometers was determined on the prece- 

 ding days, but not during the observations. The Zevenboomen 

 chronometer indicated almost exactly mean time, but the other 

 lost about 14 seconds daily. This retardation was not taken 

 into account in the former reduction ; its influence, however, is 

 found to be quite imperceptible, since it amounts to only 



14 1 



—. — ^7; = TT^th of a second per minute, which is about the 

 24x60 100 r 



time required by the velocity of transmission. 



Hence we see also that a slight alteration in the daily rate 

 would have no effect upon the result. 



The belief that the influence of wind is eliminated by means of 

 simultaneous discharges is erroneous. This is the case even 

 when the direction of the wind coincides with that of the base- 

 line, and it is so in a still greater degree when the directions 

 are not the same. 



This has been already pointed out in a paper by Professor 

 van Rees (Garnier and Quetelet, Correspondance math, et phys. 



* [These two fractions do Dot agree. If the first is correctly given, the 

 second should he jT^wTq- — Transl.] 



f r 0-023 1 ™ -. 



[ 6^433 = 3000-- TRANSL - ] 



