i8o SECTION PHYSICS. 



carried round by clockwork once in twelve hours. For the balanced 

 magnetometer the axis of the cylinder is placed vertically, but for the 

 other two instruments, horizontally ; so that the combination of the 

 motion of the point of light on the paper, with the motion of the paper 

 in a perpendicular direction presents a means of tracing out the 

 magnetic curve. Here I have an actual photograph of a magnetic 

 disturbance which took place in the observatory at Toronto, where, 

 for many years, as at other places, these instruments had been, 

 in action. The cylinder goes twice round in the twenty-four hours y 

 and therefore there will be two photographic traces on the paper. In 

 one of these you will see the changes are very small, while in the 

 other they are very large, and there is a constant vibratory distur- 

 bance of the magnet which has been maintained through nearly the 

 whole period of twelve hours. It is quite impossible that any eye 

 observations could ever have made us acquainted with the details of 

 magnetic disturbance which are shown by photographs of this kind. 

 With regard to the amount of movement, I would state to you that the 

 distance is about seven feet one inch to seven feet six inches from the 

 mirror to the point of light which falls on the photographic paper, and 

 inasmuch as the angular motion of the magnet is doubled in the 

 angular motion of the point of light upon the paper, this would 

 represent the actual amount of change which would take place in a 

 magnet of about thirty feet in length, that is, the end of it would 

 move to the extent here indicated. That is the general system of 

 photographic registration which has been adopted at Greenwich, at 

 Lisbon, in America, in Canada, and many other places. 



There is one further point to which I wish to draw your attention, 

 and that is to the mode of correcting these instruments for changes 

 of temperature. It is perfectly well known that as the temperature 

 of a magnet rises, it loses its magnetic force, and where the changes 

 are not beyond a certain amount, as it cools down again it regains 

 its power when it arrives at the same temperature from which it 

 started. It is therefore evident that the movements noticed would be 

 due to two causes, to changes in the earth's magnetism and therefore 

 to the induction upon the magnet, and also to alterations in the force 

 of the bar itself, because the position which the bar takes is the joint 

 action of the earth on the magnet, and of the magnet on the earth. 



