108 MR BROUX ON THE RELATION OF THE VARIATIONS OF THE 



to be a day or two after the new moon, and of the greatest heat a day or two 

 after full moon, in the same way as our periods of greatest cold and heat are 

 after the winter and summer solstices. This seems to agree with the periods of 

 maximum and minimum horizontal intensity. If M. Cournot be right, or if Sir 

 John Herschel's supposition be insufficient, then we must look to the solar ema- 

 nations reflected or radiated from the moon for the causes of the variations of the 

 earth's magnetism, and to our atmosphere for a cause of the supposed retardation 

 of epocli.* 



21. The connexion of lunar phase and horizontal intensity was first noticed 

 by me in July 1845. I am not aware of any investigations on the relation of the 

 horizontal intensity to the lunar month, excepting a paper by M. Hansteen, of 

 whicli I have lately merely seen the title, which refers to the connexion of the 

 horizontal intensity with the moon's ascending node. 



22. Having mentioned some time ago to Professor Forbes, that I was engaged 

 in examining the relation of the lunar periods to the variations of the earth's 

 magnetism, I learned from him that M. Kreill of Prague had stated, in his 

 volume of observations for 18-12, that the horizontal intensity was greater at the 

 moon's passage of the inferior meridian, than at its passage of the superior meri- 

 diiin. I know not whether M. Kreill has verified his statement, or to what extent 

 his observations prove it.j I have now discussed the observations for 1844, with 

 reference to this period, and have verified my results by a similar discussion of 

 the observations for 1845. I shall, at present, merely state the leading facts, and 

 leave the details to another communication. 



23. The observation at the hours on which the moon was on the meridian were 

 termed hours, the observation the hour after one hour, and so up to 24 ; as the 

 moon takes about 25 hours to return to the meridian again. On some occasions 

 there were only 24 observations between the two passages ; in these cases (few 

 in number) the hour of passage was reckoned as 24 hours, and also as hours of 

 the next day. The summations for the hours were made for each month ; I shall 

 only speak of the means for the whole year in this communication ; these means 

 have been projected. The large disturbances have been eliminated from the sum- 

 mations for 1844 and 1845. (See Curves, No. 9, Plate IV.) Any observation in 

 1844 which shewed a difference from the monthly mean, for the hour at which 



* It is evident that the variations of horizontal intensity may he due either to changes of the 

 total intensity, or of its direction ; any reasoning, therefore, on these facts must be necessarily incom- 

 plete, until we are certain of the actual effect. 



t I have, since this was written, been favoured by Professor Forbes with a copy of M. Kreill's 

 table for the horizontal force during the moon's hour angle. It indicates a minimum of intensity 

 about two hours before the meridian passage, and maximum peaks at 12'^ and 15'', giving the interpo- 

 lated period of maximum about 1 } hours after the inferior meridian passage ; the latter period agrees 

 completely with my own conclusion, the former difters about three hours from my result. 



