Ixii REPORT — 1869. 



It is believed tliat tlie results of this Table afford satisfactory evidence not 

 only of the accuracy vrith ■which the Standard Thermometers are read, 

 but also of the accuracy of tabulation from the traces. A tendency in the 

 monthly mean differences to decrease from their first values at starting wiU 

 be noticed in the case of all the observatories. 



Anemogeaph. 



The accuracy of the Anemograi^h is, like that of the Thermograph, liable 

 to be deranged by three causes : — 



(1) By a cause depending on the situation and exposure of the instru- 



ment. 



(2) By instrumental deficiency, such as fi'iction. 



(3) By deficient traces and tabulations. 



Situation of Instruments. 



These instruments are placed on the highest points of the various obser- 

 vatories, and as far as possible out of the reach of local influences. The 

 exposure may therefore be considered good in the case of all the obser- 

 vatories. 



Instrumental Deficiencies, 



Priction is the most important of these, and may be supposed to affect to 

 a small extent both the records of direction and velocit)*. The axle of the 

 direction-vane moves in a wooden bearing, which is saturated with oU. It 

 is believed that when the instrument is regularly attended to, the friction 

 consequent upon this arrangement can be kept very small. 



As regards the influence of friction upon the velocity-records, this has 

 been determined in the case of the Kew instrument, and also by Dr. Robin- 

 son for his Anemograph, which has been for many years in operation. The 

 following friction coefficient has been adopted, with the concurrence of Dr. 

 Bobinson, as applicable to the records of all the Anemographs belonging to 

 tlie Meteorological Committee : — 



Observed. miles. miles. 



For velocities from 0-0 

 to 0-5 



to 



} 



1-01 



3-0 I 



t-0-1 



)-0| 



add 1-5 

 add 1-0 



to lo^^^l-addO-r. 



Errors of Trace and Tabulation. 



It ought to be noticed that hotJi the direction- and the velociiy-pencils are 

 working well and freely on the j^aper. 



It is also to bo noticed that, for all the observatories except Falmouth, 

 the needle on the cylinder goes through the centre of the crosses marked on 

 the metallic paper. 



In Palmouth the velocity-pencil is slightly out in position, and in con- 

 seqiience that observatory has been directed to set to a point which is not 

 quite in the centre of the crosses. The Ealmouth instrument has also been 

 oriented for this position of setting. A note of the proper position of setting 

 for Falmouth is preserved at Kew, and the assistant there ought to inspect 

 each Falmouth Anemogram to see that it has been properly set. 



