EXAMINATION OF THERMOMETERS. 



Ill 



A standard barometer, wrapped in cotton-wool and cloth to prevent 

 rapid change, in the temperature of its mercury, and made by James 

 Green, is hung on the same level as the boiling-point apparatus, and 

 the thermometers are read alternately with the barometer. The 

 cathetometer is used for reading thermometers in both the boiling 

 and freezing-point apparatus. 



For the calibration of tubes, two microscopes have been mounted 

 so that the position of the two ends of a short mercury-column may be 



Fig. 3. 



t t' 



read at the same time by means of eye-piece micrometers. The observ- 

 atory is having built a comparator especially for this work, which 

 will soon be mounted in its place. 



By far the most valuable apparatus in connection with this work is 

 the collection of foreign standards which have been obtained to repre- 

 sent the work of foreign observatories. This collection comprises sev- 

 enteen standards of the highest class, eight working standards, and 

 forty-five comparison thermometers. The makers comprise noted 

 artists of Europe, and among them are the Kew Observatory; Baudin, 

 Fastre, Tonnelot, and Alvergniat, of Paris ; Fuess, and Greiner & 

 Geisslei', of Berlin ; and Casella, of London. 



The comparison of the important standards was undertaken by 

 the Kew Observatory in England, the Seewarte at Hamburg, and the 

 Imperial Commission of Weights and Measures, under Dr. Foerster, at 

 Berlin. There can be little doubt, therefore, that the observatory of 

 Yale College possesses an accurate copy of the standard thermometers 

 now in use in the prominent observatories in Europe. 



It is the object of the observatory to make this service as widely 



