184 



REPORT — 1869. 



similar to those designed for the use of the Committee by Sir Wm. Thomson, 

 and the other an extra strong Six's thermometer, as supplied to the Admi- 

 ralty by Casella. The iniiuence of great pressure on the indications of ther- 

 mometers having recently attracted considerable attention, it may be well to 

 state that the greatest pressure to which those used at Kentish Town have 

 been submitted is about a fifth of a ton per square inch, and this causes the 

 Six's thermometer to rise about 0°-4 ; Sir W. Thomson's thermometer being 

 protected by an outer glass tube is entirely uninfluenced by this pressure, or 

 even, as Professor Miller's experiments have shown, by a pressure of two tons 

 and a half on the square inch*. Hence it is certain that pressure has been 

 deprived of aU influence. The use of two thermometers of diff'erent con- 

 structions ensured the detection of any slipping or accidental error in the 

 observations, but in the regular series not a single instance of the kind has 

 occurred. 



" In order to ascertain the depth of the instruments easily, accurately, and 

 independently of any variation in the hygrometric condition of the lowering- 

 cord, it was conducted from the windlass round a grooved wheel exactly 

 86 inches in circumference, to whose axle an endless screw was attached, 

 which worked a train of divided wheels, so that the exact distance could 

 be taken at any instant. 



" It was supposed that several trustworthy obsei'vations could be obtained 

 in the course of one day ; but the following Table shows that this was not 

 the case, and confirms the expediencj-, where practicable, of allowing con- 

 siderable time for the instruments &c. to come to thermal equilibrium. At 

 Kentish Town the observations on which reliance is placed have been made 

 at intervals of not less than six days, and generally of seven. On two or 

 three occasions, however, attempts have been made to obtain observations at 

 short intervals, and the following are the results: — 



* " Professor W. A. Miller's experiments were made with an hydraulic press, and are de- 

 scribed in the Boy. Sec. Proceedings for June 17, 1869 (No. 113). Several thermometers 



