EXAMINATION OF THERMOMETERS. 



371 



ditions under which the 212 point of the English standard is deter- 

 mined under act of Parliament. 



A description of the Kew standards referred to is given in the 

 accompanying table : 



The tubes of which the Kew standards are made are about twelve 

 years old, and belong to the series purchased by the Royal Society and 

 deposited at Kew to be used as standards. 



The essential parts of the water comparator in use at Yale for com- 

 paring thermometers is shown in outline in Fig. 1, where a a' a" is 

 a bright-tinned iron cylindrical tank 15 X 20 inches, having an aperture 

 for the stopcock f, a lid a a" with various apertures for the insertion 

 of long thermometers, and having a plate-glass window 4x14 inches 

 set in the side. Within this outer tank a smaller copper tank 11x15 

 inches is symmetrically placed, and rests upon wooden bars which 

 are supported by the bottom of the outer tank. A window, placed 

 in the same relative position as the outer window, allows the ther- 

 mometers which are attached by springs at their upper ends to the 

 adjustable brass disk d', to be read. The brass axis c c' turns in a 

 bearing c', and has attached to it two disks, d, d'. A small cathetometer 

 with its telescope is placed before the, windows, and the number of the 

 thermometer under observation is shown by means of the graduated dial 

 at d. "Water of a given temperature is admitted through the tubes at e, 

 and after the temperature has been brought to the degree required, 

 it is thoroughly agitated by moving vertically the ring plunger shown at 

 ppp' p'. The disk d', which is perforated, will accommodate sixty-four 

 thermometers. The agitation of the water having subsided, the ther- 

 mometers to be compared are read as rapidly as possible, first from left 

 to right and then from right to left. Two standards are read at the 

 beginning and end of the series. It is obvious that, if they are read at 

 approximately equal intervals, the mean of the two readings will be 

 free from the error of radiation caused by the slow cooling or warming 

 of the water. The greater part of the work of the observatory upon 

 standards is done with this comparator. For clinical thermometers 

 a smaller apparatus, constructed on the same general principles, is 

 used ; but, as in this case a much less degree of precision is desired 

 than in the investigations of standards, the work may be simplified. It 

 is not necessary, for instance, to resort to the somewhat tedious read- 



