122 Trans. Acad. Set. of St. Louis. 



liglit.^ The effect on the magnetic needle at Greenwich 

 and Stonyhurst was recorded at the same instant, that 

 a solar disturbance was observed by Young (within the 

 instrumental errors). Such results should lead us to 

 expect that wind-gusts in air ionized by sunlight or by 

 solar dust, as has been pointed out by Arrhenius, should 

 produce similar results of a more local character. 



In 1823, Barlow^ made an experimental study of the 

 diurnal variation of the earth's magnetic field. He de- 

 flected the needle into an east and west position, by 

 means of two control magnets, lying in the magnetic 

 meridian, acting upon opposite ends of the needle. At 

 his request, his associate, Christie, continued the work. 

 The effect of the earth's field was in part compensated 

 by a magnet parallel to the dip needle. Their papers are 

 in sequence in the Phil. Trans, of the Eoyal Society for 

 1823. Barlow gives his conclusion as follows : 



''It appears to me that the quantity of daily change 

 depends in a greater degree on the intensity of the solar 

 light, than on the mere temperature of the day, although 

 it is certain from some recent experiments of Mr. Chris- 

 tie, that the change of temperature of the air during the 

 day, has a much greater effect upon the intensity of the 

 opposing magnets than I could possibly have imagined." 



Christie varied the temperature of the control mag- 

 nets by placing upon them paper moistened with cold 

 and with hot water. He concluded from the effect thus 

 produced upon the magnets, that temperature, if not 

 the only cause, is the principal cause of the daily varia- 

 tion in the earth's field. 



A part of the work of both Barlow and Christie was 

 done in their gardens, and the remainder in their houses. 



= The Sun. By Young, p. 158. 



' Barlow should not be forgotten. He was the first to make an electric 

 motor. Barlow's wheel is the rotating armature of an electric motor. 

 Eight years later Faraday reversed this toy motor, and produced the 

 first electrical generator. Neither of these men realized what he had 

 accomplished. 



