170 REPORT—1850. 
after overcoming some difficulties, a very perfect form had apparently been given to 
them. In alluding to this part of the operation, the Astronomer Royal took occasion 
to express his strong disapproval of the practice (so universal among engineers) of 
turning and boring with the axis of the lathe or borer horizontal. The difficulties 
in the shaping of these pivots appeared to have arisen entirely from the action of the 
weight of the iron transversely to the axis, and were overcome at last only by the use 
of counterpoises with friction-wheels : in all probability they would never have pre- 
sented themselves if the boring and turning had been effected with the axis vertical. 
The axis being thus made of cast iron, it followed almost as a necessary mechani- 
cal consequence that the telescope tubes and the circles should be made of cast iron. 
It had been matter of serious anxiety to ascertain whether the rusting of the iron, 
which might be expected at the contact of the iron with the band of silver on which 
the divisions were to be cut, would be so great as to endanger the firmness of the 
silver band. An experimental circle had therefore been cast and turned, and a silver 
band had been inserted ; and this circle had been exposed to wet in every possible 
form, till the whole was covered with such a mass of rust, that the band, in some 
places, could scarcely be seen. On clearing off the rust, it was found that the surface 
touching the silver was nowhere affected. 
On a Register Hygrometer for regulating the Atmospheric Moisture of 
Houses. By J. G. Apro.p. 
This instrument, with a variation of one-quarter of a degree in the hygrometric 
state of.the atmosphere, opens a valve capable of supplying ten quarts of water per 
hour, conveying it on to the surface of warm pipes covered with blotting-paper, by 
which the water is evaporated until the atmosphere is sufficiently saturated, and the 
valve thereby closed. 
A lead pencil attached registers the distance the hygrometer travels, and thus a 
sheet of paper moved by a clock would show the hygrometric state of the atmosphere 
at any period of time. 
The author presented a drawing and description of the apparatus. 
On an improved Door Spring. By Grorce Beatriz, Edinburgh. 
The motive power is the pressure of the atmosphere acting on one side of a pis- 
ton, the other side being a vacuum. In applying this pressure to shut a door, about 
2 105. to the square inch is lost by the friction of the machinery. The pressure of 
the air acts simply as.a counterbalance on the piston, the resistance being uniform 
throughout the travel of the door when opening it, and when shutting the door the 
regularity of motion and avoiding of slam is obtained by means of a stream of oil 
being made to discharge from a cylinder through a large or small aperture, accord- 
ing to the speed required. Fluids being almost incompressible, the oil will not pass 
through the aperture beyond a given rate, which is in proportion to the size of the 
aperture and the quantity to be discharged, and the power of the cylinder the vacuum 
is formed in to press it through. There is nothing in the machinery employed 
liable to break or get out of order. The construction was minutely described. 
The President, Str Davip BrewsTER communicated the substance of a note ad- 
dressed by Dr. Jules Guyot to the Abbé Moigno claiming the priority of the inven- 
tion of the tubular bridge. Dr. Jules Guyot had taken out patents in France in 
1844 and 1845, and one in England in 1846, for a tubular girder consisting of a 
number of hollow rectangular parallelopideds or cells, formed either of bars or 
frames (chassis) of iron united by pins, or formed of one piece of cast iron. This was 
no doubt an anticipation of Mr. Stephenson’s idea of a tubular bridge, as stated in 
1846 to the Committee of the House of Commons ; but Mr. Stephenson afterwards 
founded his claim to that invention upon his bridge at Ware, erected about October 
1843, previous to Dr. Guyot’s patent. Sir D. Brewster, however, states that a tu- 
bular girder bridge had been erected in 1840 or 1841 by Mr. A. Thomson over the 
