May Sth, 1871.] 1 20 [Heller and Brightley. 
no slight matter to the engineer, and lastly, there is no straining of the 
eyes in sighting. 
The spider’s web, by reason of its fineness, is the only article hitherto 
used for cross hairs, yet in use these have been attended with some difti- 
culties : first, the spider’s web is hygrometric, or is affected by the hu- 
midity of the atmosphere—when exposed to dampness lengthening, and of 
course throwing the line of collimation from its true place. This defect 
is more serious in the Engineer’s Levelling Instrument than in the 
Transit, instances being known where the line of collimation has altered 
two or three times in the course of ten hours, by reason of atmospheric 
changes, and of course any observations taken at those times would be 
defective ; lastly, the spider’s web being a transparent and not an opaque 
substance, in some positions it is impossible to see the hairs at all—this 
is more especially the case when sightiug in the direction of the sun; 
that is, an easterly course in the forenoon, or westerly in the after- 
noon. 
To remedy this defect, platina cross hairs , 755 of an inch in thickness, 
or as fine as spider’s web, are substituted ; these being opaque, and not 
transparent, in sighting in the direction of the sun are still visible, and 
any atmospheric changes, dampness, &c., do not affect them. We believe 
that we are the first ones,in this country who have drawn wire so thin, 
and the only ones who have made any practical use of Dr. Wollaston’s 
experiment. The platina hairs are invaluable in Mining and Tunneling 
Instruments, that are so constantly exposed to dampness, and being 
opaque, no reflector to illuminate the cross wires is required. 
To prevent the stiffness of working of the leveling, tangent and other 
screws in cold weather, which arises from the congealing of the grease 
that is used in lubricating them, no oil is used upon the screws of this 
instrument, but they are lubricated with pure plumbago. 
By a simple arrangement of the clamps on the axle of our complete 
Transits, we make them also answer the purpose of a pair of Compass 
sights, for taking offsets at right angles to the telescope. 
From the above, it will be seen.that this instrument has the following 
improvements over the ordinary Transit :—1. A simple, secure and steady 
method of attaching and detaching from the tripod, being the only long- 
centre transit made that detaches as easily as a short centre. 2. An im- 
portant decrease of weight, without decrease of size, and an increase of 
steadiness. 3. All the working parts of the tangent screw, &c., brought 
within the plates, making the instrument more compact. 4, An improved 
tangent screw, telescope, cross hairs and tripod head. 5. A pair of 
sights for taking offsets; and 6. A new method of lubricating the 
screws. 
