130 Altitudes of Mountains, &c. visible from 
bined with the oxide of lead; and that this compound, united 
‘to chromate of lead, gives rise to the red chromate, which thus 
contains a little more oxide of lead than the neutral chromate. 
A few drops of dilute nitric acid take away from it immediately 
its red colour, by dissolving the alkali with a little of the oxide 
‘of lead. I examined whether the red lead of Siberia, which is 
“also yellow when reduced to powder, might contain a portion of 
alkali; I found in it, after taking every precaution, a little lime, 
but I am ignorant whether or not it is accidental. 
XXXVI. Description of the Methods employed in determining 
the Altitudes of several of the principal Mountains and other 
remarkable Oljects visible from the Trigonometrical Station 
on Rumbles Moor, Yorkshire. By A ConRESPONDENT, 
To Dr. Tilloch. 
Pazparatory to taking the field in the spring of last year, to 
collect the requisite data for determining the abovementioned 
altitudes, I deemed it advisable, so slender was my stock of in- 
formation on the subject of terrestrial refraction, to make, during 
the winter, daily observations of the apparent altitude of a moun- 
tain, of which the elevation as well as that of the place of ob- 
servation could be readily determined by levelling. Rumbles 
Moor and an observatory 67 ,032 feet distant, both situated within 
three miles of a canal communicating with the Irish sea, were 
ultimately made choice of. 
The instruments made use of in measuring the angles, were 
two horizon sectors, of which the following is a brief description : 
The one first used consists of a 30-inch achromatic telescope 
a (see Plate II.) fixed in the hollow square frame of maho- 
gany, b; toone of its vertical sides is attached a plate of brass, 
c, containing 10 degrees of elevation and an equal quantity of - 
depression. The radius of the are is nearly 18 inches, admitting 
the divisions to be read off to 5” by the moveable index d, which 
carries with it the adjustable spirit-level e. When the line of 
vision is known to be parallel to the plane of the horizon (the 
index being at zero) the bubble of the level is adjusted to re- 
main in the middle. The small cross level f determines the 
vertical position of the divided plate, and the line of collimation 
is rendered parallel to it by means of a proof telescope. When 
the object is elevated or depressed, the corresponding angle. is 
measured by the index d, properly levelled ; that is, moved by 
the pinion (4) until the displaced bubble is again in the middle. 
Granting the interior sides of the glass level parallel to each 
other, 
