330 M. Quetelet on Atmospheric Electricity. 



of the ail' is real or only apparent ; whether the result is not 

 due, for example, to a derangement of my instrument ? or, sup- 

 posing this weakening of the electricity well ascertained, was it 

 local or not ? All these questions derive still higher importance 

 from the fact that the anomaly remarked coincided very nearly 

 with the return of that scourge which caused our population to 

 suffer so cruelly. These difficulties have led me to search whether 

 there were not means of rendering an electrometer comparable to 

 itself at different times. 



I have already shown in my first memoir that it is easy to 

 compare two Peltier's electrometers with each other, and to con- 

 struct tables of equivalents for their indications ; but when a 

 comparison has been made and the compared instrument has 

 been carried into another country, or even without, after it has 

 been employed for a certain time, it is important to be assured 

 that its indications have not varied, but have preserved their 

 absolute values. 



This verification may .be very easily made ; it is enough to 

 ascertain that the needle has preserved all its mobility, and that 

 its directive power has remained unchanged. Now the directive 

 force is given by the little magnetic needle attached to the indi- 

 cating needle of the electrometer; it is sufficient therefore to 

 submit this little needle to the ordinary processes employed to 

 determine magnetic energy, that is to say, to make it oscillate 

 freely in a horizontal plane, and to ascertain that its magnetism 

 has remained unaltered. It should be well understood that ac- 

 count is kept of the ordinary corrections employed in such cases 

 for temperature, torsion of thread, variations of the horizontal 

 intensity of the earth, &c. 



One can understand how by analogous processes we may also 

 make the determinations of the absolute electric force of the earth 

 depend on that of its absolute magnetism ; an important problem 

 with which I cannot occupy myself at this moment. It is suffi- 

 cient for me to establish, that by a very simple process we may 

 ascertain whether a Peltier's electrometer has remained com- 

 parable with itself. The thing is so simple that I am astonished 

 the precaution has not as yet been indicated. If the ideahad come 

 into my mind of thus verifying the needle of my electrometer in 

 .1849 and at preceding and following epochs, I should not now 

 have had to search out whether my instrument could undergo a 

 temporary derangement, nor to look after foreign observations 

 to control my own. 



If you think that these simple remarks may be of any service 

 x)r induce other philosophers to undertake those series of electric 

 observations so important to science, I beg you to make such 

 use of this letter as may seem to you expedient, &c. &c. 



(Signed)' Quetelet. 



