Mechanical Science. 179 



5. ON THE BUR OF PERFORATIONS. 

 (R. W. Fox, Esq.) 



If any slender and sharp-pointed instrument (a common needle for 

 instance) be made to revolve quickly whilst piercing a card, it pro- 

 duces an elevation or bur on each side of it. Hence, may it not be 

 inferred that the same effect, caused by an electrical discharge, is 

 due to the rotation of the electrical current or currents ? for the 

 edges of a hole made by electricity seem to be too regularly and 

 completely elevated on both sides of a card to be reasonably attri- 

 buted to the simple action of opposite currents not in rotation : * 

 and when several cards are thus perforated together, ought not the 

 outermost cards, on the latter hypothesis, to have "the burs on their 

 inner surfaces, and very little, if any, externally ? but it is known 

 that the elevations are generally equal on both sides of the cards: so 

 that the simple mechanical fact above stated appears to strengthen 

 the presumption in favour of the rotatory motion of electrical cur- 

 rents. 



6. CONDENSATION OP MERCURY BY PRESSURE. 



It having been deemed very interesting to employ the pressure of 

 the water at great depths for ascertaining the condensation of mer- 

 cury by pressure, the expedition commanded by Captain Kotzebue 

 was furnished with an elaterometer expressly adapted to this pur- 

 pose. It consisted of a wide thermometer tube, open at one end, 

 and having attached to it at the other a bulb like that of a thermo- 

 meter. The tube and bulb were filled with mercury, and a drop of 

 oil poured over it. A scale was attached to the tube, whose divi- 

 sions showed the thousandth parts of the whole volume of the bulb 

 and tube filled with mercury. When the instrument was made to 

 descend to great depths of the ocean, the greatest condensation 

 which the mercury had undergone during the experiment became 

 visible by the oil adhering to the inside of the tube, even after the 

 mercury had returned to its former state of expansion. At the tem- 

 perature of 19 C (66.2 Fahr.), the mercury stood at the zero point 

 of the scale. Let these degrees be = T, and those at the depth of 

 the sea = t ; the expansion, for one degree of the thermometer, of 

 mercury = m; of glass = w; and the volume of mercury at the tem- 

 perature jT= V : then the contraction of the mercury, on account 

 of the temperature, will be = V, (Tt) (mri), which is to be 

 deducted from the whole condensation observed by the instrument. 

 The following experiment was made in 21 14' north latitude, and 

 196 1' west of Greenwich, at the depth of 914.9 toises (5851 Eng- 

 lish feet). At the greatest depth the thermometer was at 2.44 C., 

 and the elaterometer marked 3.l. The condensation was, there- 

 fore, 0.0031. Having T= 19 and *=2.44, n = 0.0000274, 

 m = 0.000185, we shall find the contraction of the mercury by 



N 2 



