348 The Lamheth OhservcUory. [July, 



photography. They are printed from a standard negative plate 

 preserved in the observatory, so that identically the same test can 

 be supplied in any number of instances where instrument makers 

 may wish to avail themselves of it. 



There are two clocks in the observatory, of which one marks 

 mean, and the other sidereal time. The mean time clock is in 

 immediate and sympathetic connection with the great central time 

 standard of the Greenwich Observatory, the communication being 

 effected by electrical agency. The bob of the clock-pendulum is a 

 hollow coil of copper wire, embracing, without contact, a selection 

 of permanent magnets. The ends of the coil pass away, and are 

 ultimately connected with the Greenwich clock, which supplies 

 through the wire a current of electricity at every alternate second. 

 The movement of the clock-train is maintained in the usual way by 

 a weight ; but the accuracy of the going is secured by the magnetic 

 contrivance. Every alternate second of true mean time, the swmging 

 magnet receives a slight accelerating pull from the coil, virtually 

 converted into a magnet itself for the instant, A tell-tale magne- 

 tized needle marks the seconds by alternate sway, but always stands 

 still at the first second of every hour by the Greenwich time, and 

 starts its vibration again at the fourth second after the hour. This 

 most elegant and efficient application of electro-magnetism is due 

 to the ingenuity of Mr, Jones, station-master at Chester, and is 

 known as " Jones's Patent Controlled Pendulum." 



Meteorological instruments, in every variety, are tested at this 

 observatory ; the standard barometer has been carefully com- 

 pared with the standards of both Kew and Greenwich. The 

 divisions of the scale are also very accurately compared with a 

 measure of length finely marked into hundredths of an inch, taken 

 primarily from the preserved national standard. Aneroids are 

 tried by a very ingenious contrivance. A batch of the instruments 

 is placed in a glass-covered cylmdrical reservoir, which is connected 

 with the receiver of an air-pump, in such a way that the channel 

 of commimication is crossed by a diaphragm of porous porcelain. 

 The receiver of the air-pump is then exhausted to a certain degree, 

 and the exhaustion of the reservoir containing the aneroids goes on 

 through the porous diaphragm very slowly, in order to imitate the 

 action of the instrument when used to measure mountain ascents. 

 The exhaustion takes place at the rate of about one inch of pressure 

 per hour, and at every half-inch of change the instrument is com- 

 pared with an accurate mercurial barometer forming part of the 

 apparatus. 



Thermometers are proved in the freezing and boiling points by 

 immersion in melting ice and in steam; and a certain number of 

 intermediate points of temperature are also verified, by comparison in 

 water, with standards authenticated at Greenwich and Kew. With 



