318 
On the Construction of a Portable Barometer. 
[Nov. 
The cistern and supply of mercury being got rid of by the syphon form, the 
triangular board may also be dispensed with. For the security of the glass tube, 
a similar wooden case will be required as in the case of the straight one, formed in 
the same way, of thin deal boards attached by screws to two square dies, and secu- 
red in the middle by small screws or nails; or as I think might answer better, a 
square case of tinned iron, which should he painted inside and out : this would be 
perhaps stronger, and certainly as light as the wooden case, while it would 
be of smaller dimensions, and therefore more convenient. As the shorter 
leg of the syphon is only inches while the longer is 32£, l would have 
this case made broader below to the above length, and narrower above; in 
fact made to fit exactly the instrument : the use of this difference of di- 
mensions will be afterwards seen, Fig. 3. Whether made of wood or of iron, the 
front should open for the purpose of inspecting the tube. A ring may be so 
affixed to the case, that when hung up with the front open, it shall be perpendicular. 
As the hanging position is not however favorable to observation, and as in very 
many situation.s there may be no convenient support to hang it on, I would propose 
to hare three stays attached to it, by means of which it may be not only steadily 
supported, but also adjusted in a perpendicular position ; a silk thread with a bullet or 
other small weight being attached inside the case, to indicate the correctness of the 
adjustment. These stays may be either of wood ; or hollow, of tinned iron ; and they 
may lie so fashioned, that when put together they will form a prismatic rod of two 
feet in length, and of such dimensions as to fill up exactly the deficiency in the upper 
part of the case, thus completing a uniform prism. (Fig. *1.) They may be attached by 
means of rings or hollow shoulders fastened to the barometer case in a way so obvi- 
ous, that any attempt at explanation would only render it obscure. The bottom of 
the case, would rest on the ground, as well as the supports or stays, and thus render 
the barometer perfectly firm. In order the better to distinguish the surface of the 
mercury and edge of the sight, the inside of the case should be painted black. 
A barometer such as I have described, and so fitted up, will 1 am confident give 
more satisfaction than any that has yet appeared in the market. 
D. 
Postscript. 
Since writing the above paper, I have met with the following notice in a late num- 
ber of the Revue Encyclopediquc, of an improvement in the syphon barometer 
which I think is well worthy of notice ; I shall therefore make no apology for offering 
the passage to your readers. 
i, * V|^i Bavart ilnd Arago made a report on a new kind of barometer proposed 
by M Bunten. Since the first application of the barometer to the measurement of 
heights, the ingenuity of observers, as well as of instrument makers, has been applied 
to modify it in a thousand different ways, and this chiefly with the view of render- 
mg it more portable. Amongst these modifications Gay Lussac's appears to us one 
i , ,a i ) P* cs l : *‘ Its lightness, its convenience, the accuracy of which it is suscep- 
tible, have been fully appreciated. We must however admit, that in particular cir- 
cumstances, it is liable to derangement; and that on foot, on horseback, and more 
especially m a carriage, it the position of the barometer be horizontal, bubbles of 
ai r are likely, nay certain, to intrude into the larger column. This is the defect 
winch M. Bunten has proposed to remove, and which he has accomplished without 
sacrificing any ot the advantages of M. Gay Lussac’s invention. For this purpose 
ie constructs the barometer in two pieces, joined afterwards together, the upper one 
being drawn out into a capillary tube, which is inserted within the lower ; through 
winch capillary tube the mercury in its ascent as well as descent must necessarily 
pass. But a bubble of air, if it make its appearance, as it always follows the inner 
sui ace of the tube, is at length stopped by the re-entering angle formed at the junc- 
tion of the two tubes ; reversing the instrument for carriage, the bubble naturally 
escapes. This improvement has removed the only objection to which Gay Lus- 
sac s barometer was liable, and this without rendering it more fragile.” 
I5y a curious coincidence, soon after readingtheabovedescription,Ilmdanoppor- 
lprw y »n , e w a,n,n,ng one of these barometers made b y Bunten, and I must acknow- 
n n a I ly . answers “»y idea of a perfect barometer. The instrument 
Zll- , y wel1 finisbed > contrary to the general opinion on this subject, which 
Irp LvIlipd tL S - 0n i e palllatlon forthe Angling and expensive contrivances which 
are applied to instruments of science, as fabricated by London artists, in the ela- 
* The syphon tube with capillary bend. 
