Heater Question

Hi folks,
Just setting up my 24l tank and I've put the filter at one side and the heater vertical on the other side, with the flow from the filter directed towards it. The thing I've noticed is that the heater seems to be switching on for a couple of minutes, then off for a couple of minutes, and its been doing this for two hours now. Its a marina 50w mini heater and I've set it to roughly 22 degrees. Should I have the heater set horizontally to stop it going on and off every couple of minutes?
Added a PIC. Also just noticed too, after letting it run for just over two hours the temp has gone up by nearly two degrees.

Some heaters do badly fully upright, as the heat that they create then turns off the thermostat. Shouldn't be a problem if there's plenty of flow over it.
 
It's worth noting that the thermostats on most heaters are, at best, approximate in their guide numbers.
 
When they're heating a tank to start with they will tend to spend a lot of time going on and off though, it's only possible to truly understand what they're doing once the tank is up to temperature.

like dr rob said they can only be understood when they are up to temprature