ITS ERRORS AND CORRECTIONS, ETC. 489 
4th, From ill-closed boxes, currents of air, and entry of spiders. Examples 
of these are unfortunately not wanting. 
5th, From the thermometer within the box not showing the true temperature 
of the magnet. The error due to this cause may be great; and it is considerable 
in observatories where stoves have been used, and where there are necessarily 
sudden changes of temperature. 
6th, From the copper ring used as a quieter, which has sometimes changed 
its position, and which has a variable effect in different positions of the magnet. 
7th, From the shifting of the telescope on its pillar. 
8th, From the approach of magnets, or articles containing iron. 
All these, and other sources of error, have to be cared for, and the corrections 
to be applied, wherever the errors exist, if it is wished to give the observations 
any real scientific value. 
