88 



Mr. F. Galton on Verifying 



[Mar. 15, 



is shown by dotted lines in the drawing. A cock at the lower end of 

 the coil permits of the stoppage of the circulation of the water through 

 the pipe. 



(4) The Thermometer Frame. 



The thermometers to be compared are hung side by side round the ' 

 circumference of two brass rings, 10| in. in diameter, that are attached 

 to the side rods of a cylindrical frame. The thermometers are held in 

 their places against the ring by spring clips, one of which is shown full 

 size in fig. 4, and one of the rings is shown in fig. 3 ; the latter slide up and 

 down the brass rods that form the sides of the cylindrical framework, 

 and are clamped at such a distance apart as may best suit the ther- 

 mometer under examination. 



Forty thermometers can be suspended at a time. The bottom of the 

 frame is provided with six rollers — three placed radially, for the purpose 

 of guiding it up and down the interior of the water-vessel ; and three 

 tangentially and projecting below the base, in order to support the 

 frame whilst putting on or taking off the thermometers. This operation 

 is performed when the frame is standing on the closed lid of the box, a 

 circular brass ring being screwed to the lid to prevent the frame running 

 off when being turned round by the operator. 



The top of the frame consists of spokes radiating from a hollow 

 socket that drops over the end of an upright steel rod, which, passing 

 through the axis of the agitator, projects above it (see fig. 2). A plug is 

 then screwed into the top of this rod and clamps the frame, which is 

 supported by it ; the frame, with the thermometers, can then be rotated 

 in the water by turning this rod. This turning is effected from the out- 

 side through a wheel fixed to its projecting lower extremity, into 

 which an endless screw, driven by a crank in front of the apparatus, 

 is geared. The observer, facing the glazed slit, can bring the ther- 

 mometers hung round the frame before him one by one as quickly as 

 he likes. 



For the convenience of moving the thermometer frame into and out of 

 the water-vessel, a cord is carried over the apparatus round pulleys, as 

 seen in fig. 1, so that the attendant can hook its end to the ring at the 

 top of the frame, and twist or lower it with the greatest facility. 



The general character of the process of comparison is to turn down 

 the gas and to close circulation in the pipe by turning the stopcock ; the 

 water is then agitated, and is afterwards left at rest until the set is 

 finished. The thermometer frame is turned once round forwards and 

 once backwards in each process of comparison, each instrument being 

 read off twice, the mean of the two being the result aimed at. 



Mr. Whipple, the Superintendent of the Observatory, has made at my 

 request a large number of experiments on the variations of temperature 

 under different conditions, and on other matters relating to the working 



