The satisfaction arising from these experiments is sometimes abated, from the circumstance that the quantity of electricity obtained in this way, is so small that its quality cannot be ascertained. To remedy this inconvenience Cavallo and Nicholson, have invented machines which they denominate _doublers_ or _multipliers_ of electricity. But the structure of these machines is complex and delicate, and the explanation of them is long, and not easily understood without the aid of plates. Our epitome therefore does not admit of inserting them. Those who may choose to pursue the subject we refer to the writers above mentioned. To prevent the inconvenience arising from wind and rain in the use of the atmospheric electrometer, the following device has been used by Mr. Cavallo.—Take a glass vessel open at top and bottom—cement it at bottom to a convenient piece of wood—let the upper part be tapering like the neck of a phial, and cement into it a glass tube, extending a little above and a little below the neck of the larger vessel. Cover the tube with sealing wax, both within and without the neck of the vessel, so as to give it the appearance of one body. Into this tube cement a brass wire extending a very little below the bottom of the tube, and flattened at the lower end so as to be perforated with two small holes. Through these holes insert two flaxen threads, or two very fine silver wires, with small balls of cork or pith at the end of them, and touching each other:—if wires are used they should be suspended by small rings at the top, that they may act more easily. Let the top of the brass wire screw into a brass cover on the top of the whole vessel, which cover will not only secure the vessel against rain, but serve as a conductor to a very slightly electrified atmosphere—conveying the fluid, first to the wire, and by means of that to the balls, which will exhibit, within the vessel, the state of electricity collected from the atmosphere. There should be two narrow slips of tin foil stuck to the inside of the glass vessel, and communicating with the wooden bottom, which will serve to carry off that electricity which, when the corks touch the glass is communicated to it, and which, if accumulated, would disturb the free motion of the corks. An useful alteration of this electrometer was made by Mr. Bennet. It consists of slips of gold leaf or silver leaf, instead of the corks suspended by threads or wires. These slips of leaf are to be suspended from the cover of a cylindrical vessel, and hanging within it. The slips of leaf are to be about two and an half inches long. This electrometer is the most sensible instrument of the kind, manifesting in an unequivocal manner very small quantities of electricity. But this instrument is not as portable and easily managed as the other.—If very fine threads, stiffened with glue, be used without any balls, they will be found nearly as sensible as the gold leaf.