228 Prof. Oersted on the Law qftJte 



endeavoured to divide, by the eye, the concave part into two 

 parts of equal volume, but the i - esults show that we have at- 

 tributed too little of this volume to the inclosed air. With- 

 out this error the differences would have been smaller. In 

 other respects, they are as small as could be expected in expe- 

 riments where a vernier could not be used. 



In the last observation, for example, where the observed 

 length of the column of air was 56.4 millimetres, the length 

 which would have entirely agreed with the law of Mariotte, is 

 56.287, which differs only 0.113 millimetres from observation, 

 an error which is unavoidable in such observations. In the 

 last experiment but one, the observed length of the column of 

 air was 63.17 millimetres, whereas the number which would 

 have satisfied the law of Mariotte, is 63.99. This difference 

 amounts to 0.82 millimetres, but, being between two other ob- 

 servations whose deviations are very small, it cannot affect the 

 general law. 



In order to examine the compression of air by great forces, 

 we made use of air-guns. Our sovereign, whose enlightened 

 magnanimity has so often contributed to the progress of 

 science, put at our disposal all the apparatus which was ne- 

 cessary for this inquiry. The reservoir of air in this kind of 

 arm is the culasse, which ought to be very strong ; we at first 

 measured the capacities of these by weighing them when emp- 

 ty, and when filled with water. 



It was then easy to calculate the quantity of air which such 

 a reservoir contained. That which we most frequently used 

 contained 0.891 grammes of air when the barometer was at 

 0.76. We were thus capable of determining, by the balance, 

 the degree of compression which we had attained in our expe- 

 riments. This method was found sufficiently exact, as the 

 balance which we commonly used was sensible to a centi- 

 gramme. We succeeded in introducing into this reservoir as 

 much as 101.2 grammes of air, a quantity equivalent to the 

 pressure of 110.5 atmospheres. We have also, in these ex- 

 periments, taken into consideration the dilatation which the 

 interior pressure ought to produce in the reservoir. This 

 dilatation was determined by weighing in water the reservoir 

 when empty, and when filled with air; and in the calcula- 



