168 Mr. Barlow on the daily Variation of the [March, 



return ; although when I vibrated it towards night, it commonly 

 took up its morning position. I made these observations with 

 the needle in various directions, viz. with the face of the instru- 

 ment to the east, west, north, south, Sec. but in every case I 

 obtained the same sort of daily motion. The question, there- 

 fore, respecting the law of variation of this instrument, still 

 remains to be submitted to fixed principles, although there can 

 be no longer any doubt that it is subject to a daily change. 



On a curious Anomaly observed between the daily Variations in- 

 doors and in the open Air. 



I have already mentioned that I was, at the commencement 

 of my experiments, a good deal embarrassed and delayed by 

 certain anomalies which I noticed between the daily changes of 

 the needle made in the house and in the garden. These maybe 

 stated shortly as follows. That in certain positions of the needle 

 towards the east and west, the daily motion, although it pro- 

 ceeded with the same determinate uniformity in both cases, yet 

 it took place in different directions ; passing in the one instance 

 from the east, or west, towards the south, and in the other 

 towards the north, at the same corresponding hours of the day, 

 the motion in both instances being equally distinct, regular, and 

 progressive. 



After carefully examining every circumstance that might be 

 supposed to be the cause of this singular change, I could only 

 imagine three, that seemed in any way likely to account for it. 



1. Were the two magnets and the compass needle in the two 

 cases in precisely the same relative situation ; and if not, might 

 not the cause lie in this discrepancy ? 



2. The window of the room was to the northward ; was it 

 possible that the light, arriving at the needle in this direction, 

 was the cause of the change ? 



3. There was an iron stove in the room ; could it be that this 

 was subject to a periodic increase and decrease of magnetic 

 power ? 



In order to examine the first of these cases, I measured very 

 carefully the distance, direction, Sec. of the compass and mag- 

 nets while in the garden, and placed them in precisely the same 

 relative situation in the parlour ; still the motion in the two cases 

 was reversed. 



To examine the second, it occurred to me that if the direction 

 of the motion depended upon that of the light, the needle ought 

 to be wholly stationary in the dark, or when excluded from the 

 solar rays. I therefore kept my room shut for two days, and 

 only examined the needle by the light of a wax taper ; but 

 although there was certainly less motion on those days than 

 usual, yet I could come to no satisfactory conclusion ; but I 

 still think that further observations will show that the solar 



