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details of these new observations, obtained by varying the inclination of the incident rays and the position of the plane of analyzation, and by employing different metals as the reflecting surfaces. By the application of the undulatory theory of light to the circumstances of the experiments and the resulting phenomena, the law of metallic retardation is made the sụbject of analytic investigation. A polariscope of peculiar construction, of which a description is given at the conclusion of the paper, was employed in the experiments; and tables are subjoined of ihe numerical results of the observations. Ibid.






Vernier Slide-Rule for taking Exact Measurements. SIR.—Having had occasion to make some measurements with minute accuracy in circumstances where a common rule could not be conveniently applied, and not finding in the tool-shops any modification of it that suited my purpose, I contrived one for myself; and as it appears to me to be a great improvement on those in common use, I subjoin a sketch, not doubting that it will be received with pleasure by many of your readers.

The annexed figure represents a 12 inch rule with a slide-the numbering of the inches on the rule itself running upwards, and those on the slide returning in the opposite direction, so that the extent of interval between the extreme ends when the slide is drawn out, is shown at the intersection of the rule and slide. At the top of the rule, a space, A, B, equal to ninetenths of an inch is divided into ten equal parts, and thus forms a vernier, enabling us to read off the length to the hundredth of an inch. In the diagram the dis

tance between the ends is above 16:1 inches, and un112 der 16.5 inches, the coincidence of 9 on the vernier

with a graduation on the rule, shows it to be 16-19 inches. The degree of minuteness in subdivision can easily be increased at pleasure to any extent that the accuracy of the workmanship and the nicety of the observations may render desirable.

The principle can be readily applied to any form of rule and seems particularly well suited for measuring the internal diameter of a tube or cylinder. The length of a line is thus determined not by the 'application of the thumb-nail or the scratch of a piece of iron or other such rude method, but by the coincidence of the divisions of the scale itself, and is thus evidently susceptible of much greater precision.

This kind of rule is so simple and so much siipe

rior to those in common use; its principle is, moreover, so obvious, that I was very much surprised to find none such already constructed. A vernier usually slides upon a scale, and here the scale slides upon the vernier; this inversion of their relative positions is the only novelty in the present arrangement.

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A rod with a slide in it and graduated like this rule, would, I think, be an improvement in the poles used for leveling; the sights might then be directed, not to a graduation on the pole, but to a fixed object on the slide, specially designed for that purpose, and which might be elevated or depressed at pleasure, by a racked wheel, and its height seen by the intersection of the rod and its slide.

Glasgow Prac. Mec. & Eng. Mag.

On the Advantages of working Engines with High-pressure Steam

expansively, and at great Velocities. By J. G. BODMER. The author based his observations upon the principle of a considerable area of piston being essential for taking advantage of the initiative impulse of highly elastic steam, in contradistinction to the idea of a percussive action, which had some time ago found advocates.Proc. Ins. Civ. Eng.

London Athenæum.

Siemen's Chronometric Governor. The action of this governor is so sensitive, that no variation of the speed of an engine, when 40 per cent. of its load is thrown off, can be observed, for the entire change is performed in one-fiftieth of the revolution of the fly-wheel; this change absorbs or adds a portion of the momentum of the pendulum, and slightly alters its arc of vibration, the limit of which is between 19° and 21°; and by the laws of pendulous motion, this is shown to effect the number of revolutions to the amount of only 8 per cent. of its velocity, and even that small variation in the extreme position of the pendulum ceases immediately the momentum is restored to its former condition.


Description of Rowan's Churn, as Manufactured by Mr. RICHARD

ROBINSON, Lisburn. Fig. 1 represents the entire churn when set up for work: it consists of an oval-shaped vessel, divided, in the direction of its longest diameter, into two compartments; in one of which the paddle-wheel works: in the other the butter is collected, as will be better understood by fig. 2, which is a section of it, viewed from above when the cover of the wheel is removed. By turning the paddle wheel (which resembles the one used in the old box-churn) a current of the milk is maintained in the direction of the arrows. As soon as the butter collects on the top of the milk, it is borne along with the current; and by sliding down the sluice (a) to the surface of the milk, the butter is intercepted, and accumulates on one side of the sluice; it is then taken out with a small tin scoop, pierced with small holes, through which any milk may pass; but perhaps the accompanying description which is sent with each churn will tend to explain it better than anything I can write :

Fig. 1.

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“ This machine possesses decided advantages over every other hitherto in use, inasmuch as it is more easily worked, and produces more butter of a better quality, than that obtained from, or by, any other churn. As to the superior quality of the butter obtained, it arises partly from the low temperature at which the operation can be performed; for while in other close machines the temperature rises during the operation, in this, the fluid being exposed to the current of air created, the temperature is found to be lower at the latter end than at the beginning of the process; besides, the butter is not so much beaten and toughened by repeatedly passing under the blades as in other machines. It is found, therefore, from all these causes united, that the quality and quantity of the butter are improved, and the labor decidedly lessened. In using a thermometer, this machine possesses superior convenience for making a true observation of the temperature; for, in other machines, the process must be stopped to try the heat; in this, the thermometer may be suspended constantly in the smaller division of the churn, and the temperature accurately observed, at any time, whilst the process of churning is going on.

« The large churns are made, in a most substantial manner, of oak and sycamore, being the best sorts of timber known for the purpose. The smaller sizes are made of strong block tin, with a separate watercase stand, so that all the advantages of the celebrated water-case churns are combined in this new and effective machine.

“ The following table is submitted as being convenient for determining the size of churns to be ordered :

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“ This table is only offered as a guide to the sizes of churns requisite, not as containing information on any other point; for quantities of cream or milk will vary much, according to food and management, &c. It is better, however, to order the churns too large than too small."

Although good workmanship and neatness of finish is an important feature in every well constructed implement, yet in no department is { this more necessary than in those utensils intended for the dairy.

Lond. Farm. Mag.

Clyburn's Self-Recording Dynamometer. The appearance of Clyburn's Dynamometer, as our engraving shows, is that of a small box on wheels. Within the box are a right and left hand spiral spring, placed within each other for economizing space. When the power is applied, the springs are compressed : whilst being compressed, motion is given to a rack and pinion; and, by a simple mechanical connection, motion from the rack and pinion is communicated to a traversing pencil upon the top of the box; the pencil there acts upon a roll of paper, marked with lineal divisions, both length ways and crossways, each division in the length of the paper indicating a distance of twenty-five yards of ground passed over, and each division of the width 112 pounds of force applied to the instrument. Motion is given to the paper by the pair of wheels which travel over the ground, whilst the pencil is moved backwards and forwards according to the amount of power applied, and thus a drawing is made upon the paper, showing the draught, and the distance traveled over.


The subjoined diagram will describe the nature of the drawing made by the pencil.



The figures down the side, from 0 to 7, represent cwts.; those of the top, distances of twenty-five yards; from which it will be seen that, according to the drawing, the force applied in draught during the fifty yards, averaged rather more than 4 cwt.; because the intersecting line from 4 shows more of the drawing towards the line 5, than it does towards that of the line 3.

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