942 



temperature ; whence the followii>g law. The pressure of steam 

 generated in the usual steam-boiler is directly proportional to the 

 fourth power of its temperature, when measured by a true scale. 



It being assumed that 100 degrees is the temperature of steam 

 when its pressure is in equilibrium v/ith a column of 30 inches of 

 mercury, or with the pressure of one atmosphere, then F being the 

 pressure in atmospheres, at any temperature 



Viooy 



A comparison is instituted between theoretic experiments of the 

 Academy of Sciences and the results of this formula, from which it 

 appears that the temperatures deduced from the formula are inva- 

 riably in defect, the greatest difierence being 3'5l, and the least 

 2-02. 



2. " On the means adopted in the British Colonial Magnetic Ob- 

 servatories for determining the absolute values, secular change, and 

 annual variation of the Magnetic Force." By Lieut.-Col. Edward 

 Sabine, R.A., For. Sec. R.S. 



The determination of the mean numerical values of the elements 

 of terrestrial magnetism in direction and force at different points of 

 the earth's surface (the force being expressed in absolute measure, 

 intelligible consequently to future generations, however distant, and 

 conveying to them a kilowiedge of the present magnetic state of the 

 globe), and the determination of the nature and amount of the se- 

 cular changes which the elements are at present undergoing, are, as ' 

 the author states, the first steps in that great inductive inquiry by 

 which it may be hoped that the inhabitants of the globe may at some 

 date, perhaps not very distant, obtain a complete knowledge of the 

 laws of the phenomena of terrestrial magnetism, and possibly gain 

 an insight into the physical causes of one of the most remarkable 

 forces by which our planet is affected. 



After stating the inadequacy of the instruments originally pro- 

 posed by the Royal Society, to the attainment of all the objects for 

 which they had been designed, the author refers to the modifications 

 which had been introduced, in the instruments and methods of ob- 

 servation for the determination oT the absolute values, and the secu- 

 lar changes of the horizontal component of the magnetic force. He 

 then gives the series of the results of the monthly observations at 

 Toronto from January 1845 to April 1849 as relatively correct ; and 

 from this series, regarding each monthly determination as entitled 

 to equal weight, and taking the arithmetical mean of all the values 

 as the most probable mean value, obtains 3*53043 as the mean value 

 of the horizontal force at Toronto, with a probable error of +'00055; 

 and the probable error of +'0040 for each monthly determination. 



This is on the most simple hypothesis, in which neither secular 

 change nor annual variation is supposed to exist. The monthly re- 

 sults however distinctly indicate a secular change, and by means of 

 them, on the hypothesis of a uniform secular change, the author 



