Thei'mometers at the Kew Observatory. 



231 



The mean variation of temperature during a double process of 

 comparing each of twenty sets of thermometers, each set averaging 

 nineteen instruments, and each instrument being read four times, 

 vras as follows : — 



Temperature at which the 1 j ^^qq 

 comparison was made, j ; 



70° 



80° 



90° 



100° 

 ±0-09 



110° 



! 

 Mean variation during 1 i n.ng 

 each set J 



±0-07 



±0-06 



+0-07 



- 



±0-15 



The extreme variation of 0°'30 occurred in one case, and that of 

 0°'25 in three cases. 



It takes about four minutes to read a complete set of ordinary 

 thermometers. 



The rate of heatiug by gas, and of cooling after the gas has 

 been wholly turned oif, is of course much affected by the 

 temperature of the air of the room ; it may be roughly taken as 

 follows : — 



i 

 When the water in the vessel is about ... 



45° 



100° 



150° 



200° 



Rise of temperature in r> minutes when 1 

 the gas is turned fully on J 



2° -80 



2°-26 



l°-95 



l°-45 





Fall of temperature in 5 minutes when 1 

 the gas is turned wholl}' off J 



1 



1 



j 0°-25 



i 



0°-60 l°-00 



1 



The rate of cooling is much reduced when the process consist® 

 in first raising the water to the highest required temperature, and 

 then cooling it by successive additions of cold water. The heat 

 of the stuffing that surrounds the vessel being thus much higher 

 than the water it contains, keeps it at an equable temperature. 



The temperature of the water in the vessel, after agitating 

 it and allowing it to settle, differs somewhat at different levels ; 

 this is due to the impossibility of securing perfect intermixture 

 and to the variations of the temperature of the stuffing in respect 

 to that of the water. The greatest differences observed between 

 a thermometer whose bulb was immersed 2 inches below the 

 level of the water and one that was immersed 19 inches was 0°-68, 



March 22.— Dr. J. Dalton Hooker, C.B., President, in the Chair. 



The following paper was read : — 



" On Stratified Discharges.™ lY. Stratified and Unstratified 

 Forms of the Jar-Discharge." Bv William Spottiswoode, M.A., 

 Treas. B.S. 



It is well know that if a Leyden jar be discharged through a 

 vacuum-tube, the discharge generally takes the form of an unbroken 

 column of light, extending from the point of the positive terDiiual 



