Description of the Marine Transit. 403 



which has been graduated according to Fahrenheit, and 

 is made to resemble, as much as possible, the gauge D, in 

 every circumstance which might affect iisresuh ; the scale 

 is jrreat, being about | of an inch to a degree, which again 

 is divided into tenths ; and the bulb is kept immersed in a 

 body of mercury, contained in the bottom of the glass case, 

 which is to be closed at the same time as the machine, and 

 like it to be well secured from the intrusion of the external 

 atmosphere. In the drawing but little of this thermometer 

 can be seen. 



Much of the accuracy of the machine will depend on the 

 lube C, which communicates with the regulator and the 

 gauoe. It is about six inches in length, of considerable 

 thickness, and its interior capacity such, that the volume 

 from j is nearly three minutes in filling it. On its trout 

 there arc marked small segments of circles 1, 2, 3, 4, 5, 6, 

 7, b; directly on the opposite side there are as many other 

 srguicntp, corresponding with the f>ame, each to each. Tl.e 

 sight of the observer nm«t be directed so, that by the re- 

 flection of the glass, two corresponding segments appear a 

 perfect circle. VViien the transit of the rising column of 

 mercury interrupts this circle, the instant shown by a se- 

 cond watch is written do-.\n by an assistant, in the same 

 way as is usual in taking celestial altitudes or distances. 

 This transit of the column of mercury can be observed to 

 the fraction of a second of time, n 



The cocks y and x, are both air and mercury tight, and 

 arc so connected by the parallel bars, that tlie one cannot 

 move independently of the other. When the ivory handle N 

 i'^ u]), as repreEcnted in the drawin<r, the lower cock z is 

 shut, and y is open. By putting the handle down, y 

 closes, and z opens ; by which simple movement, having 

 prtvioui-ly lowered the elastic lube froin the hocik IV^ the 

 mercury in the gauge D, discharges itstlf into the gUjhe £, 

 from whence it is restored to the magazine AA, by drawing 

 a rope fastened to the ring /?, and passing througli a pulley 

 fixed above in the deck. In this operation the globe E, 

 attached to the rods hh, ascends, forming a circular sweep, 

 and carrying with it the rods /<//, wlucli turn upon their 

 joints Jf. When the globe E has been raised in this 

 manner to the necessary height, the mercury runs from 

 it into the maga/^ine A A, through the pipe ooo; which of 

 course is ilien elevated. 'J'hc globe h] being eir.ptied, it 

 niuxl be lit gradually down again to its place. A couple 

 of prongs i and t, attached to the framing at the bottom of 

 the gauge D, embrace the rods by wliieh it is hung, and 

 C t 2 lock 



