^€$ Oil tlie Mvafurt »f «..,5^V Ihporturi. 



pafpa, but In this it i« pnlji^ requ!](ed to ke^pt thf orifice perpendicular to cTi& HotI^oo^ 

 though it be prefented to the line of motion in an oblique dire£lion. ,, .,, 



If the horizontal tube of fuch an inftrumect be pJaced, by means of the diredtittg rod 

 and index, in a tn»e faft pr >f?ft dire^iqiij according as the fliip maVes eailipg or^ vyeftingt 

 (which direction coniequently muft be altered »s often qs the fhip's courfe is alt?jed.).th9 

 quantify of water difgharged thereby will be pxopoifUwwte t9. tite departure, which appear* 

 fropi the foJlowing confideraitions : 



When the plane of the orifice of thiiioftruintnt coincJdcfrwLth the plane ojf a meridian* 

 If the fliip moves in a due eaft or w?ft courfe, the dire£t impa£l of the water will occafion 

 8 difcharge from the orifice, which meafures the fpace paffed over (as was (hewn in the 

 caCe of the difiance infi:runient) but in failing on a parallel circle this (pace ia the 

 departure. 



Again, if the (hip fails along a meridian with the orifice of the tube pointed as before, 

 the ftream will be parallel to the plane of the orifice, and therefore will occafion no pref- 

 fure on it, or difcharge of water from the aperture, in which cafe alfo the departure is 

 reprefented by the re.fult, becaufe a fhip failing on a meridian makes no departure. 



And laftly, if this inflrument (the orifice being flill in the plane of a meridian) be moved 

 •long a rhumb-line, the refiflance which the lowed plate of water at the orifice n^eets, is 

 the fan)? as that whiph, the orjfice of a fimilar inftrument would meet in paffing over the 

 departure of that rhumb in a direft pofition, and in the fame time, becaufe the prefTure on 

 each orifice is as the produdl of the force and number of impinging particles which ftrikes 

 it; but the force of a fingle particle on the diretl orifice, is, to the force of a particle oifi 

 the obliq_ue orifice in a ratio compofed of the ratio of their velocities, that is, as departure to 

 i'lflapce^ and. of the r^tio of radius to the IJne of the angle of obliquity, that is, 7\.$- diftance 

 to departure ,- therefore thefe forces are in a ratio compounded of the direft and inverf^ 

 ratios of the fame pair of quantities, which is the ratio of equality, and every particle 

 llrikeseach orifice with equal force. 



Again, the number of particles which (Irikes thefe orifices is the, fame, becaufe they con- 

 ftitute two homogeneous columns, which having the fame bafe, and lying between the 

 fame parallel planes are equal, whence It follows, that the prefTures on thefe orifices are 

 equal at all times^ and confequently the quantities of water difcharged through thefe aper- 

 tures in equal times are equal alfo ; but fince the quantity difcharged from the direfl; in- 

 ftruipent would meafure the fpace pafled over by it, which is the departure, it is evident 

 that the quantity difcharged frpm the oblique inflrumetit in the fame time mufl give the- 

 departure alfo, and in general that the fpace given by fuch an inftrument in an oblique, 

 pofition, is as the fine of the angle of obliquity ; hence it is evident, that if this inftrumerit; 

 be ufed with its orifice pointed to the north or fouth, inftead of the eaft or weft, that, \% 

 will (hew the difference of latitude, which furnifhes the beft method of trying the pcr- 

 loinfiance of the inftrumetit ;, fpr if when, ufed ip.this H^ape at fea, the difference of latitude; 



