158 $C(VTCH nsHBRIES, 



.!Defcriptkm of I do not djfguife the con fi deration, that as the oil dijninifliej^ 

 a nt-vv ftdtical (|,g djftances between the upper and lowet furfaces of the 

 denfie fluid mufl; diminilh, and a proportional difference or 

 fubftdence in the furface of the oil muft take place. The 

 proper remedy for this appears to be that the lower furface 

 diould be made as large as convenience will allow ; that its 

 rife and fall may be lefs. 



With regard to the difpofition and form of the fpaces which 

 are fo contain the oil, it is only needful to obferve that they 

 may ail be made fmall or narrow, except that which is alter- 

 nately to be occupied by the oil, and the denfe fluid. If the 

 height of the denfe fluid be 12 inches, the lamp may fland 

 18 or 20 inches high, uling fait water as above mentioned. 

 There are various pradiical objeftions to mercury ; but if 

 this fluid were to be ufed, the oil might be raifed ten times as 

 high, or the apparatus, if required, might be conftrufted with 

 a lefs diftance between the furfaces.* 



I am. Sir, 



Your confiant Reader, 

 ■''''■■ ■ A. F. 



XVI. 



Litter from a Conejpondent re8ifi/ing fome Particulars of Mifi. 

 information refpeBing the Fijlicry of the North of Scotland. 



To Mr. NICHOLSON. 

 SIR, 



W HEN any in^portant information is communicated ta 

 the public, we have a right to expedl that it fliould be given 

 with extreme accuracy; or at leaft where any doubt^ exift, 

 with fuch a degree of diffidence and noodefty, as may leave 

 room for avoiding mifreprcfentation or falftiood. 



* The contrivance for keeping a fluid at its level by a feml-cir- 

 cular revolving folic! was invented by Robert Hooke. See Birch's 

 Hiftoiy of the Royal Society. A. F. has ingenioufly adopted it t6 

 a lamp which cafts no fhadow. Hooke's lamp is nearly as faulty 

 as the common fountain laitip in this refpeft. N, 



