504 Mr. R. Hosking on the 



o 



the arm, and opposite a telescope ; the other could be moved 

 along the arm and clamped in front of the water surface. 

 When the mirrors had been set, it was thus possible to read 

 off the positions of the two water surfaces by means of the 

 telescope, at the same instant; for both images were arranged 

 to be side by side in the field. The pressures employed 

 varied between the limits 100 cms. (water) and 42 cms. 

 (mercury). 



The time of flow was in most cases very short, the average 

 being about one minute, but in extreme cases it was as low 

 as 22 seconds. Special means had to be employed to register 

 the time intervals correctly. The chronograph used was 

 kindly supplied by the Sydney Observatory. It consisted 

 of two electromagnets side by side. The armatures were 

 provided with needles. Paper tape was fed through rollers 

 immediately over the needle points at the rate of about 

 5 cms. per second. A special spring enabled the needles to 

 travel forward a little on piercing the tape, and prevented 

 the tearing of the tape. One needle was used for recording 

 seconds by direct reference, through electrical contacts, to a 

 standard clock. The other needle punctured the tape when 

 a key was pressed at the transits of the meniscus in the 

 glischrometer at the points a and b in the one case ; or at c 

 and d in the other. These transits were observed always 

 through telescopes. 



The procedure in determining the viscosity was as follows. 

 The bath temperature was arranged to be as close to the 

 desired temperature as possible, and the heating flame was 

 adjusted. The pressure of air in the reservoir was raised 

 or lowered to the proper level. Double readings of pressure, 

 time of flow, and temperature were taken. The pressure 

 was next altered, and more readings were taken. In most 

 cases, the determinations were repeated. Another capillary 

 was then placed in position in the glischrometer and the 

 series was repeated. 



The Reduction Formula. — In the Journal and Proceedings 

 of the Royal Society of New South Wales are published 

 three most important papers by G. H. Knibbs*, dealing 

 with the history, theory, and determination of the viscosity 

 of water by the efflux method. Knibbs has shown that the 

 reduction formula is 



ttR 4 , t m.$.V(l + 2*0 J 



v*= ? r\^- t 7^ HV' • (1) 



8 V L( 1 + n ~J SttLI 1 + n £ JT 



* Vols, xxix., xxx., and xxxi. 



