46 Dr. Ballot on the importance of Deviations^/row the mean 



it has led, in connexion with the observation of the wind, to 

 practical discoveries. Who knows not the fruits of the labours 

 of Redfield, Reid, Piddington, Thorn ? In later times also, 

 the investigations of Mr. Birt on the atmospheric waves, which 

 in November pass over England and Europe, are only de- 

 duced from the knowledge of the deviations. 



As we must give so much importance to the deviations of 

 the barometer and thermometer as to assert that they are ap- 

 plicable in most meteorological investigations ; as we further 

 consider how much trouble it causes for a calculator to sub- 

 tract the value obtained from every observation which he uses 

 from the mean value at that place for that day, whilst the 

 observer can so very easily, whilst he is noting his observa- 

 tions, note in a column next to it the difference from the mean, 

 we dare propose and urgently invite the observer everywhere, 

 where the mean of each day is sufficiently known, or even 

 where this is known with some approximation, to take this 

 little trouble. We should then, after a lapse of five or ten 

 years, be able to modify the mean of the temperature, and 

 then to determine it with greater certainty. A book that con- 

 tains such means for every day of the year for many places 

 would certainly be a most useful book ; and if one once pos- 

 sessed that, then it would be sufficient for the observers to 

 communicate the deviations ; from time to time the correc- 

 tions could be published in the form of supplements. 



Art. IV. England has given the example of communicating 

 the state of the weather* at a certain instant in all parts of the 

 country, at least to Glasgow, by means of telegraph, as I saw 

 to my surprise in the Greenock Daily News, My propo- 

 sition made in the Changements de Temperature was actually 

 in operation. Later I have also learned that that plan is like- 

 wise proposed to the British Association : thus the communi- 

 cation of the weather becomes more scientific than it hitherto 

 had been. Telegraphs can, better than self-registering instru- 

 ments, answer the purpose of approaching to a foretelling of 

 the weather at all places before it exists there ; they give us 

 the opportunity of being informed of the weather before it has 

 passed from one place to another. We can be on our guard 

 and arrange our occupations (our observations in the first place) 

 accordingly; remarkable phenomena will be noticed by dif- 

 ferent observers in corresponding manners at different places, 

 and many other advantages will result. It is true, we shall 



* This publication was principally planned and is carried into execution 

 by the proprietors of the London Daily News. The report of the state of 

 the weather at 9 h in the morning at numerous places is published every 

 day in that journal. — G. B. Airy. 



