2 Description of a new Transit Instrument. 
fixed in a window or other opening in a common dwelling- 
house ; and that it is quite impossible, excepting in fixed obser- 
vations, to make it sweep the entire arch between the southern 
and northern points of the horizon. In windy weather its use is 
also very difficult and inconvenient. In the instrument now 
offered, all these inconveniencies are avoided. It may be fixed 
almost any where; in many places it may be made to describe 
the entire semicirele of the meridian; the observer is put to no 
difficulties by change of place, as he always looks directly along 
the axis; it is packed in one eighth of the space.requisite to 
pack a common transit instrument of the same real size; its 
weight is not more than a sixth of the other ; from its simplicity 
it will be afforded at half the price; and its verifications and 
adjustments are easy and simple. It has also another advantage— 
that the mark by which it is placed in the meridian may be 
either in the meridian or at right angles to it ; or, if convenient, 
two marks may be erected, one to the south or north, the other 
east or west ; and if so used, it will be always seen by inspection 
only, whether the mirror needs adjustment or not. In many 
confined situations, such as occur in cities, the power of having 
a mark at right angles to the meridian may be eminently useful. 
The general description of the instrument is as follows : 
The telescope is included in a brass cylinder having a small 
cylinder at each end, turned true in the usual manner, and resting 
in Ys of the usual construction. These smaller cylinders are 
both pierced. In one is the eye- piece of the telescope with its 
wires, &c. The other is open for the purpose of seeing through 
it, if necessary, the eastern or western mark, and for adjusting 
by direct vision the line of collimation of the instrument. It 
also serves for the illumination of the wires. The object-glass 
of the telescope is placed so near this cylinder as only to allow 
room for an unsilvered plane glass mirror to be placed before it, 
at an angle of 45°. It is obvious that, as the telescope revolves 
on its axis in the Ys, every celestial object at right angles to it 
will successively be seen by reflection from the mirror; and, of 
course, if the axis be placed due east and west, the transits of all 
celestial bodies over the meridian will he observed with the ut- 
most accuracy and convenience. The aperture in the axis be- 
yond the object-glass is not only of use for the adjustment of the 
instrument to an eastern or western mark, or for the illumination 
of the wires, but affords a means of seeing the mark at the same 
time with the body whose meridian passage is to be observed, 
and of thereby being certain of the true adjustments of the m- 
strument at the very moment of observation ; which is impossible 
in any other construction of the transit instrument, and seems 
to be a very material advantage. . 
Tam 
