Mechanical Science. 171 



The copper rod, of which the dilatation is exactly known, would 

 have made the index describe 301 mm. if it had been fixed to a 

 body insensible to caloric. He deduces that the 69.16 mm. of 

 difference is due to the stone. These experiments, repeated alter- 

 nately on different stone bodies with a rod of copper, and with one 

 of iron, have given the following results : 



Absolute Dilatation. 



Brass 0.00187821 



Soft hammered iron .... 0.00122045 

 Carrara marble .... 0.00084867 

 Marble from St. Beat . . . 0.00041810 



Solst .... 0.00056849 



Pierre de Vernon .... 0.00043027 

 St. Leu .... 0.00064890 



M. Destigny, wishing to know if the hygrometric state of the 

 stone would produce any change in its dilatation, exposed to 

 humidity a stone from St. Leu, and found that the dilatation was 

 the same. One fact surprised him in this experiment, — it was the 

 enormous quantity of water absorbed by the stone, almost amount- 

 ing in volume to one-third of the stone itself. 



The difference between the expansion of iron and stone accords 

 with what was before known relative to the bad effects, and their 

 causes, occasioned by introducing iron in certain situations into 

 stone buildings. M. Destigny proposes to use compound and com- 

 pensating bars of iron and copper for these purposes, but we doubt 

 whether practice will sanction the suggestion. 



15. New Artificial Horizon. — There are few more cumbersome 

 instruments than the artificial horizon, in the manner in which it is 

 usually constructed, so that an improved one, which we have seen 

 with Mr. Newman, whose character for manufacturing the best 

 philosophical apparatus is known to every friend of science in this 

 country, promises to be of material advantage to the traveller. 

 This instrument is comprised in a square box, the top of which is 

 detached when the instrument is in use. The lower part contains 

 the mercury, which is raised into the brass basin above by means of 

 two screws, which previously kept together the parts of the box, 

 acting upon a moveable bottom. The basin is rubbed over with 

 nitrate of mercury, an invention of Professor Schumacher, ofAltona, 

 the effect of which is to make the quicksilver adhere to the sides of 

 the basin, so that, with a little care, a perfectly level surface may 

 be obtained, free from tremor, and well suited for the purpose for 

 which it is required. By turning the screws, the quicksilver is 

 then let down again into the reservoir ; and the whole fulfils that 

 indispensable condition in instruments designed for a traveller, 

 that " the box shall not close unless ail-that it contains is locked, or 

 the act of closing locks them." 



