322 Kew Observatory. 



require to know the relative diameter of the bore at all the 

 different parts of the tube. In order to obtain this informa- 

 tion, a small portion of mercury, sufficient to occupy about 

 half an inch of the bore, is detached from the main body of 

 the fluid in the bulb by a mechanical process, and is made to 

 travel down the tube from the bottom to the top, its length 

 being accurately measured at every stage. Of course where 

 the bore is wide the length of this detached comma will be 

 small, and where the bore is narrow, its length will be great. 

 By this method the diameter of the bore is ascertained through- 

 out, and the instrument graduated accordingly. 



The result of the labours of the Kew committee was soon 

 apparent. The slovenliness with which meteorological instru- 

 ments had hitherto been constructed gave place to accuracy, 

 and such are now produced by many opticians with hardly any 

 perceptible error. But here let me impress upon all those who 

 desire perfection not to remain content with the general repu- 

 tation of the optician whom they employ, but to have their 

 instruments verified at Kew, and a table of corrections pro- 

 cured from that establishment. By doing so, not only is the 

 instrument itself rendered practically equal to a standard, but 

 the optician is kept up to the mark by the knowledge that his 

 work is scrutinized. It is now time to notice shortly the 

 various scientific processes which are conducted at the obser- 

 vatory. That meteorological observations are regularly made 

 at Kew, our readers are well aware ; and here we may likewise 

 mention the fact that Robinson's anemometer has been im- 

 proved by Bukly, the mechanic of the observatory, into an 

 instrument which records continuously the direction and 

 velocity of the wind, and which is now extensively adopted. 

 But perhaps the most important processes are thoso connected 

 with photography. Light plays a very prominent part at Kew. 

 By menus of this agent, the changes which take place in the 

 magnetism of our globe, as well as those which take place in 

 the electricity, and the pressure of the atmosphere, are continu- 

 ously recorded, and, besides all this, the sun is made to take 

 his own likeness. 



We cannot here enter into details of construction, let us 

 rather inform our readers what such instruments have already 

 achieved, and what more they may be expected to accomplish. 

 Of the self-recording instruments ai Kew the magnetographs 



are perhaps the umst, important, and the records of these in the 

 hands of General Sabine have already led to very interesting 

 results. Our readers may be surprised to learn that nothing in 

 nature is more inconstant than the magnetic needle; not only 

 has it a motion depending upon the hour of the day, but it has 

 likewise a change from season to season, and from year to year. 



