40 1 Description of the Marine Transit. 



lock 11 securely; so that while locked it can have no mo- 

 tion indcjiendeiit o) the machine. 



The beiit tube in tim regulator B is an air pipe, for the 

 putpose of maintaining a ^ir'ic circulation of the air con- 

 tained in tiie apparatus, whether atmospheric or any other; 

 should it be f;)und expedient to close the machine without 

 adopting the Torricellian vacuum. A similar tube is fixed 

 in the niagaznie, and for the same purpose. 



With this knowledoe of the several parts, the following 

 wili be sufficient instructions for conducting the operation 

 on ship board : 



On leavintij a known meridian, unhook the tube mynm, 

 and put down the ivoiy handle iV. While the mercury is 

 running out of the gauge into the globe, be careful to 

 compare, with the time of the day, "the instants of the 

 transit of the rising cohimn of mercury over the segments 

 1, ?, 3, 4, t^c. Having recorded these observations, and 

 the gauge l)eing perfectlv enipiv, put up the ivory handle, 

 raise the glc)be £, and let the mercury that is in it run into 

 the magazine ^-lA. 



On the morrow when the c;aii!ie is full, and the mercury 

 lias appeared rising above the cock y, put down the han- 

 dle N to cut ofl the communication, and unhook the elastic 

 tube as before. Now carefully observe again the colunrn's 

 transit over the segments I, 2, 3, &c.; and together with 

 these observations write down the height af the large ther- 

 ntometer M. The gauge being empty, put up the ivory 

 handle, and restore the mercurv in the globe E to the ma- 

 gazine yiA. By reference to the tables showing the capa- 

 city of the gauge at evc-ry tenth of a degree, find against 

 the temperature, which was noted down, the time that has 

 elapsed since the last operation ; which, compa>-fd with the 

 ship's time, jrives the loi.gitude sought, or the ship's de- 

 parture from the meridian of t!ie preceding day. Record 

 this time in your book of reckoning, and proceed from day 

 to day in the same maimer. 



If the operator should be too late, and find the graduated 

 tube C filled, ne will have lost the opportunity of observing 

 his time for that. day. The chamber G, which the aper- 

 ture ;' would be about an hour and a half filliusi, will se- 

 cure the instrument from iioing down ; if, in allusion to a 

 watch, I njay be permiired the expression. On (^uch an 

 occasion it will be only requisite to unhook the tube, put 

 down the ivory handle, and register the time found in the 

 tables to correspond with the height of the large thermome- 

 ter. Then, wheu the gauge is empty, put up the ivory 



handle. 



