42 Mr. William Galbraith, on some 



perature 56°*5, and stirred with a thermometer ; the solu- 

 tion was rapid but incomplete. The thermometer sunk 

 from 56°-5 to 51° or 5°^. 



4. 300 grains of crystallized proto-sulphate of iron in 

 powder, were thrown into 1000 grains of water of the tem- 

 perature 58°, and the mixture was stirred till the salt dis- 

 solved. The thermometer sunk from 58° to 53°* 5 or 5° J . 

 So that the cold evolved by the solution of sulphate of 

 magnesia and proto-sulphate of iron is sensibly the same. 



The quantities of water of crystallization in 300 grains of 

 each of these salts are as follows : 



grains. 



Carbonate of soda . . . 187*50 

 Sulphate of soda .... \QQ'QQ 

 Sulphate of magnesia . . 153*65 

 Proto-sulphate of iron . . 135*96 

 Now, the ratios of these numbers to each other are very 

 nearly as the numbers 37 J, 33^, 30f , 21 1. 



While the cold produced by the solution of each salt 

 was 16°, 12°, 5°^, 5°i. 



We see that these two ratios are not the same or even ana- 

 logous to each other. It is obvious from this that the mere 

 knowledge of the water of crystallization, and the solubi- 

 lity of a salt, is not sufficient to enable us to foretell the 

 degree of cold that will be induced by its solution in water. 

 A great deal depends upon the rapidity of the solution. 

 Hence, it happens that more cold is produced by dissolving 

 salts in dilute acids ; because by this method the rapidity of 

 the solution is very much increased. 



Article VII. 



On some Astronomical Methods of Observation. By William 



Galbraith, a. M., Teacher of Mathematics^ Edinburgh. 



I. ON THE OBLIQUITY OF THE ECLIPTIC. 



To trace the various methods of astronomical observation 

 used by the ancients, would be a task too laborious and irk- 

 some for our present purpose. It would not, however, be 

 uninteresting to notice a few of their processes and instru- 

 ments which they most generally employed. Among the 

 latter the gnomon constructed in various ways appeared to 

 be that in which most confidence was placed. 



