458 Analyses of Books. [June, 



needle from its direction (at least there is only one exception to 

 this, which will be noticed hereafter), and consequently all the 

 effect produced upon the needle during the rotation of the index 

 in the circle N E S W, is due to the vertical wire only, except 

 so far as the horizontal wires may increase or diminish the direct- 

 ive power of the needle. This, however, in the cases to which 

 we shall refer, is very inconsiderable." 



" But in order that we may know precisely what part of the 

 change of deviation between one situation of the compass and 

 another, is actually due to that change of position, recourse 

 must be had to the standard compass, which, always remaining 

 fixed in its position, may be used as a constant indicator of the 

 strength of the battery. But as the application of this measure 

 to computation is involved in principles not at present explained, 

 it will be proper first to inform the reader of the means which 

 I employ in the first instance to preserve an uniformity of action 

 during every separate course of experiments. These were as 

 follow : — 



" The vessel which contains the dilute acid, into which the 

 plates are immersed, holds nearly 20 gallons ; and I begin the 

 experiments with little more than 12 gallons ; moreover the 

 plates are not, in the first instance, let down to their lowest 

 point. The intensity shown by the standard compass after the 

 connexion has been made some minutes, is noted ; and by 

 breaking off and making the contact anew, this same intensity 

 occurs again, the power being always strongest when the con- 

 tact is first made ; then when the standard compass returns to 

 its former bearing, the observation with the other compass is 

 taken ; the contact broken and renewed, and so on as long as 

 the battery retains sufficient power. When this fails, the plates 

 are lowered a little more ; the power thus increased, and the 

 observations resumed, till at length the plates being wholly down, 

 and the power too weak, recourse is had to a supply of more 

 dilute acid ; by which means a tolerably steady action is kept 

 up longer than is necessary for any series of experiments of this 

 kind. It will be observed here, that in this case the only use 

 made of the standard compass is to indicate the same intensity 

 of action, and consequently involves no theoretical principle that 

 will be objected to by the most scrupulous theorist or observer, 

 but it will be seen in subsequent articles that this indicator is 

 susceptible of a more extensive application." 



Prof. Barlow having thus described his method of experi- 

 menting, now proceeds to explain the principles of computation, 

 and to compare the numerical results thus obtained, with those 

 derived from experiments. 



" According to the hypothesis, if we conceive the wire in the 

 first instance to be vertical, and the compass placed to the 

 north or south of it, and opposite its middle point, the centre of 

 action will lie in the horizontal plane, and at right angles to the 



