188 REPORT—1850. 
The motion in right ascension is given by a screw working in a portion of a circle, 
or rather large flange, stretching round the cylinder, except on the telescope and de- 
clination face, in a plane at right angles to the polar axis. 
For setting the instrument in right ascension, there is a small but complete circle 
turning on, and concentrically with, the upper pivot, but independent of it, and 
constantly kept in motion by clock-work. 
On a Mode of Cooling the Air of Rooms in Tropical Climates. 
By Prof. Prazzi Smyru. 
The author proposed to himself some plan by which to effect in a warm country, 
the reverse of what is effected in a cold country, by the simple operation of lighting 
a fire. 
The case is shown to be a signally important one by the sufferings and early deaths 
of our countrymen in India, and no methods at all touching the real question at 
issue have yet been invented ; and certainly all such partial and incomplete plans 
would fail, if tried in the following circumstances,—in a country where the tem- 
perature by day and by night, and by summer and winter is never under 90°, where 
the water and earth are as hot as the air, and where the atmosphere is saturated 
with moisture. ‘ 
But however untoward these conditions may appear, it was shown that by the 
following method the air might be cooled down to any desired degree. 
Compress the air in a closed vessel, the air will rise in temperature, say from 90° 
to 120°; keep it in the compressed state until the heat of compression shall have 
been dissipated by radiation and conduction, and then allow it to escape, when it 
will sink as much below its original temperature as it rose above it on compression, 
or will issue at 60. 
The above is hardly anything more than merely a statement of an old fact long 
known, and particularly illustrated in the ancient Schemnitz machine; the author 
only claimed the merit of applying this property of air to so useful a sanitary pur- 
pose, of determining the quantity of compression required to produce a given altera- 
tion of temperature, and of contriving a convenient form of machine for practical 
purposes. The principle is said to have been applied last year in America to making 
ice ; and in the beginning of this year Sir J. Herschel sent a reclamation of priority 
to the Athenzum in favour of a suggestion of his own to the same effect, but neither 
have given any determination of the exact amount of the thermotic effect of com- 
pression on which the practicability of the plan for the ordinary purposes of life 
must rest; while the author of this paper can go back as far as 1844 for the date of 
an apparatus which he had constructed to test the point, and to the beginning of 
1849 for the first publication on the subject. 
Towards the end of last year he had an unusually good opportunity afforded him 
of testing the subject experimentally on a very large scale by the kindness of Mr. 
Wilson of the Thinniel iron-works. From these experiments, it appeared that a 
compression of one-quarter of an atmosphere produces an elevation of 29° to 30° F. 
in the temperature of air at 60°. The determination is likewise borne out theoreti- 
cally by Carnot’s. theory of heat, and by that of Mr. Macquorn Rankine, produced 
last winter; avd he has further computed that one horse power working one hour 
will cool 9000 cubic feet of air 20° F., without allowing anything for friction and 
mechanical imperfections. ᾿ 
Making all due allowances for these drawbacks, Prof. Smyth had estimated from 
his experiments that a pair of bullocks (the most convenient and available source of 
mechanical power in India) should furnish 70 cubic feet of air per minute, cooled 
20° below the surrounding atmosphere ; and that the expense of thus cooling a house 
in a warm country would not be more than that of warming a house in a cold one, 
and might be managed as efficiently and completely. Y 
On the Application of Telescope Sights to Rifles. By Prof. Ptazzi Suyrn. 
The ordinary plain sights of rifles are attended with four inconveniences :— 
Ist. There are three objects to be brought in a line, the sight at the breech, that 
