Hat box collapsible version for evenings, known as a gibus. They might also include storage space for items such as a hat brush. Designs became quite large during the Victorian era. A letter to "The Times" in 1844 warned travellers that Blackwall Railway's porters had charged a 1d to carry a hat box onto the train and a further 6d for transporting it to the London terminus, with the traveller himself paying only 4d for the journey. He recommended that travellers with luggage should go by steamboat. While traditionally hat boxes are circular or square in shape, some versions may follow the
Top-hat filter theoretically possible. Top-hat filters can be constructed by chaining theoretical low-band and high-band filters. In practice, an approximate top-hat filter can be constructed in analogue hardware using approximate low-band and high-band filters. In Fourier space, a top hat filter selects a band of signal of desired frequency by the specification of a lower and upper bounding frequencies. Top-hat filters are particularly easy to implement digitally. The top hat function can be generated by differentiating a linear ramp function of width formula_1. The limit of formula_1 then becomes the Dirac delta function. Its real-space form is the same as the moving